diff --git a/libcxx/docs/Status/RangesAlgorithms.csv b/libcxx/docs/Status/RangesAlgorithms.csv
--- a/libcxx/docs/Status/RangesAlgorithms.csv
+++ b/libcxx/docs/Status/RangesAlgorithms.csv
@@ -27,8 +27,8 @@
 Search,search_n,Not assigned,n/a,Not started
 Search,find_end,Not assigned,n/a,Not started
 Read-only,is_partitioned,Nikolas Klauser,`D124440 <https://llvm.org/D124440>`_,✅
-Read-only,is_sorted,Not assigned,n/a,Not started
-Read-only,is_sorted_unitl,Not assigned,n/a,Not started
+Read-only,is_sorted,Nikolas Klauser,`D125608 <https://llvm.org/D125608>`_,✅
+Read-only,is_sorted_unitl,Nikolas Klauser,`D125608 <https://llvm.org/D125608>`_,✅
 Read-only,includes,Not assigned,n/a,Not started
 Read-only,is_heap,Not assigned,n/a,Not started
 Read-only,is_heap_until,Not assigned,n/a,Not started
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -81,6 +81,8 @@
   __algorithm/ranges_for_each.h
   __algorithm/ranges_for_each_n.h
   __algorithm/ranges_is_partitioned.h
+  __algorithm/ranges_is_sorted.h
+  __algorithm/ranges_is_sorted_until.h
   __algorithm/ranges_max.h
   __algorithm/ranges_max_element.h
   __algorithm/ranges_min.h
diff --git a/libcxx/include/__algorithm/ranges_is_sorted.h b/libcxx/include/__algorithm/ranges_is_sorted.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/ranges_is_sorted.h
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP__ALGORITHM_RANGES_IS_SORTED_H
+#define _LIBCPP__ALGORITHM_RANGES_IS_SORTED_H
+
+#include <__algorithm/ranges_is_sorted_until.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/projected.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+namespace __is_sorted {
+struct __fn {
+  template <forward_iterator _Iter, sentinel_for<_Iter> _Sent,
+            class _Proj = identity,
+            indirect_strict_weak_order<projected<_Iter, _Proj>> _Comp = ranges::less>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  bool operator()(_Iter __first, _Sent __last, _Comp __comp = {}, _Proj __proj = {}) const {
+    return ranges::__is_sorted_until_impl(std::move(__first), __last, __comp, __proj) == __last;
+  }
+
+  template <forward_range _Range,
+            class _Proj = identity,
+            indirect_strict_weak_order<projected<iterator_t<_Range>, _Proj>> _Comp = ranges::less>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  bool operator()(_Range&& __range, _Comp __comp = {}, _Proj __proj = {}) const {
+    auto __last = ranges::end(__range);
+    return ranges::__is_sorted_until_impl(ranges::begin(__range), __last, __comp, __proj) == __last;
+  }
+};
+} // namespace __is_sorted
+
+inline namespace __cpo {
+  inline constexpr auto is_sorted = __is_sorted::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+#endif // _LIBCPP__ALGORITHM_RANGES_IS_SORTED_H
diff --git a/libcxx/include/__algorithm/ranges_is_sorted_until.h b/libcxx/include/__algorithm/ranges_is_sorted_until.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/ranges_is_sorted_until.h
@@ -0,0 +1,76 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP__ALGORITHM_RANGES_IS_SORTED_UNTIL_H
+#define _LIBCPP__ALGORITHM_RANGES_IS_SORTED_UNTIL_H
+
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/projected.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+
+template <class _Iter, class _Sent, class _Proj, class _Comp>
+_LIBCPP_HIDE_FROM_ABI constexpr
+_Iter __is_sorted_until_impl(_Iter __first, _Sent __last, _Comp& __comp, _Proj& __proj) {
+  if (__first == __last)
+    return __first;
+  auto __i = __first;
+  while (++__i != __last) {
+    if (std::invoke(__comp, std::invoke(__proj, *__i), std::invoke(__proj, *__first)))
+      return __i;
+    __first = __i;
+  }
+  return __i;
+}
+
+namespace __is_sorted_until {
+struct __fn {
+  template <forward_iterator _Iter, sentinel_for<_Iter> _Sent,
+            class _Proj = identity,
+            indirect_strict_weak_order<projected<_Iter, _Proj>> _Comp = ranges::less>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  _Iter operator()(_Iter __first, _Sent __last, _Comp __comp = {}, _Proj __proj = {}) const {
+    return ranges::__is_sorted_until_impl(std::move(__first), std::move(__last), __comp, __proj);
+  }
+
+  template <forward_range _Range,
+            class _Proj = identity,
+            indirect_strict_weak_order<projected<iterator_t<_Range>, _Proj>> _Comp = ranges::less>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  borrowed_iterator_t<_Range> operator()(_Range&& __range, _Comp __comp = {}, _Proj __proj = {}) const {
+    return ranges::__is_sorted_until_impl(ranges::begin(__range), ranges::end(__range), __comp, __proj);
+  }
+};
+} // namespace __is_sorted_until
+
+inline namespace __cpo {
+  inline constexpr auto is_sorted_until = __is_sorted_until::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+#endif // _LIBCPP__ALGORITHM_RANGES_IS_SORTED_UNTIL_H
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -305,6 +305,22 @@
    constexpr bool ranges::equal(R1&& r1, R2&& r2, Pred pred = {},
                                 Proj1 proj1 = {}, Proj2 proj2 = {});                        // since C++20
 
+  template<forward_iterator I, sentinel_for<I> S, class Proj = identity,
+           indirect_strict_weak_order<projected<I, Proj>> Comp = ranges::less>
+    constexpr bool ranges::is_sorted(I first, S last, Comp comp = {}, Proj proj = {});      // since C++20
+
+  template<forward_range R, class Proj = identity,
+           indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
+    constexpr bool ranges::is_sorted(R&& r, Comp comp = {}, Proj proj = {});                // since C++20
+
+  template<forward_iterator I, sentinel_for<I> S, class Proj = identity,
+           indirect_strict_weak_order<projected<I, Proj>> Comp = ranges::less>
+    constexpr I ranges::is_sorted_until(I first, S last, Comp comp = {}, Proj proj = {});   // since C++20
+
+  template<forward_range R, class Proj = identity,
+           indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
+    constexpr borrowed_iterator_t<R>
+      ranges::is_sorted_until(R&& r, Comp comp = {}, Proj proj = {});                       // since C++20
 }
 
     constexpr bool     // constexpr in C++20
@@ -1036,6 +1052,8 @@
 #include <__algorithm/ranges_for_each.h>
 #include <__algorithm/ranges_for_each_n.h>
 #include <__algorithm/ranges_is_partitioned.h>
+#include <__algorithm/ranges_is_sorted.h>
+#include <__algorithm/ranges_is_sorted_until.h>
 #include <__algorithm/ranges_max.h>
 #include <__algorithm/ranges_max_element.h>
 #include <__algorithm/ranges_min.h>
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -313,6 +313,8 @@
       module ranges_for_each          { private header "__algorithm/ranges_for_each.h" }
       module ranges_for_each_n        { private header "__algorithm/ranges_for_each_n.h" }
       module ranges_is_partitioned    { private header "__algorithm/ranges_is_partitioned.h" }
+      module ranges_is_sorted         { private header "__algorithm/ranges_is_sorted.h" }
+      module ranges_is_sorted_until   { private header "__algorithm/ranges_is_sorted_until.h" }
       module ranges_max               { private header "__algorithm/ranges_max.h" }
       module ranges_max_element       { private header "__algorithm/ranges_max_element.h" }
       module ranges_min               { private header "__algorithm/ranges_min.h" }
diff --git a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
@@ -134,10 +134,10 @@
     //(void)std::ranges::is_partitioned(a, UnaryTrue(&copies)); assert(copies == 0);
     //(void)std::ranges::is_permutation(first, last, first2, last2, Equal(&copies)); assert(copies == 0);
     //(void)std::ranges::is_permutation(a, b, Equal(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted(first, last, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted(a, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted_until(first, last, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted_until(a, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted(first, last, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted(a, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted_until(first, last, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted_until(a, Less(&copies)); assert(copies == 0);
     //if (!std::is_constant_evaluated()) { (void)std::ranges::inplace_merge(first, mid, last, Less(&copies)); assert(copies == 0); }
     //if (!std::is_constant_evaluated()) { (void)std::ranges::inplace_merge(a, mid, Less(&copies)); assert(copies == 0); }
     //(void)std::ranges::lexicographical_compare(first, last, first2, last2, Less(&copies)); assert(copies == 0);
diff --git a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
@@ -117,10 +117,10 @@
     //(void)std::ranges::is_partitioned(a, UnaryTrue(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::is_permutation(first, last, first2, last2, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::is_permutation(a, b, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted(first, last, Less(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted(a, Less(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted_until(first, last, Less(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::is_sorted_until(a, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted(first, last, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted(a, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted_until(first, last, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::is_sorted_until(a, Less(), Proj(&copies)); assert(copies == 0);
     //if (!std::is_constant_evaluated()) { (void)std::ranges::inplace_merge(first, mid, last, Less(), Proj(&copies)); assert(copies == 0); }
     //if (!std::is_constant_evaluated()) { (void)std::ranges::inplace_merge(a, mid, Less(), Proj(&copies)); assert(copies == 0); }
     //(void)std::ranges::lexicographical_compare(first, last, first2, last2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
diff --git a/libcxx/test/libcxx/private_headers.verify.cpp b/libcxx/test/libcxx/private_headers.verify.cpp
--- a/libcxx/test/libcxx/private_headers.verify.cpp
+++ b/libcxx/test/libcxx/private_headers.verify.cpp
@@ -118,6 +118,8 @@
 #include <__algorithm/ranges_for_each.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_for_each.h'}}
 #include <__algorithm/ranges_for_each_n.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_for_each_n.h'}}
 #include <__algorithm/ranges_is_partitioned.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_is_partitioned.h'}}
+#include <__algorithm/ranges_is_sorted.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_is_sorted.h'}}
+#include <__algorithm/ranges_is_sorted_until.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_is_sorted_until.h'}}
 #include <__algorithm/ranges_max.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_max.h'}}
 #include <__algorithm/ranges_max_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_max_element.h'}}
 #include <__algorithm/ranges_min.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_min.h'}}
diff --git a/libcxx/test/std/algorithms/alg.sorting/alg.sort/is.sorted/ranges.is_sorted.pass.cpp b/libcxx/test/std/algorithms/alg.sorting/alg.sort/is.sorted/ranges.is_sorted.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.sorting/alg.sort/is.sorted/ranges.is_sorted.pass.cpp
@@ -0,0 +1,178 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// <algorithm>
+
+// template<forward_iterator I, sentinel_for<I> S, class Proj = identity,
+//          indirect_strict_weak_order<projected<I, Proj>> Comp = ranges::less>
+//   constexpr bool ranges::is_sorted(I first, S last, Comp comp = {}, Proj proj = {});
+// template<forward_range R, class Proj = identity,
+//          indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
+//   constexpr bool ranges::is_sorted(R&& r, Comp comp = {}, Proj proj = {});
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <functional>
+#include <ranges>
+
+#include "almost_satisfies_types.h"
+#include "test_iterators.h"
+
+template <class Iter, class Sent = Iter>
+concept HasIsSortedIt = requires(Iter iter, Sent sent) { std::ranges::is_sorted(iter, sent); };
+
+struct HasNoComparator {};
+
+static_assert(HasIsSortedIt<int*>);
+static_assert(!HasIsSortedIt<ForwardIteratorNotDerivedFrom>);
+static_assert(!HasIsSortedIt<ForwardIteratorNotIncrementable>);
+static_assert(!HasIsSortedIt<int*, SentinelForNotSemiregular>);
+static_assert(!HasIsSortedIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
+static_assert(!HasIsSortedIt<HasNoComparator*>);
+
+template <class Range>
+concept HasIsSortedR = requires(Range range) { std::ranges::is_sorted(range); };
+
+static_assert(HasIsSortedR<UncheckedRange<int*>>);
+static_assert(!HasIsSortedR<ForwardRangeNotDerivedFrom>);
+static_assert(!HasIsSortedR<ForwardRangeNotIncrementable>);
+static_assert(!HasIsSortedR<ForwardRangeNotSentinelSemiregular>);
+static_assert(!HasIsSortedR<ForwardRangeNotSentinelEqualityComparableWith>);
+static_assert(!HasIsSortedR<UncheckedRange<HasNoComparator*>>);
+
+template <class Iter, class Sent = Iter>
+constexpr void test_iterators() {
+  { // simple test
+    {
+      int a[] = {1, 2, 3, 4, 3};
+      std::same_as<bool> decltype(auto) ret = std::ranges::is_sorted(Iter(a), Sent(Iter(a + 5)));
+      assert(!ret);
+    }
+    {
+      int a[] = {1, 2, 3, 4, 3};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      std::same_as<bool> decltype(auto) ret = std::ranges::is_sorted(range);
+      assert(!ret);
+    }
+  }
+
+  { // second element isn't sorted
+    {
+      int a[] = {1, 0, 3, 4, 5};
+      auto ret = std::ranges::is_sorted(Iter(a), Sent(Iter(a + 5)));
+      assert(!ret);
+    }
+    {
+      int a[] = {1, 0, 3, 4, 5};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      auto ret = std::ranges::is_sorted(range);
+      assert(!ret);
+    }
+  }
+
+  { // all elements are sorted
+    {
+      int a[] = {1, 2, 3, 4, 5};
+      auto ret = std::ranges::is_sorted(Iter(a), Sent(Iter(a + 5)));
+      assert(ret);
+    }
+    {
+      int a[] = {1, 2, 3, 4, 5};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      auto ret = std::ranges::is_sorted(range);
+      assert(ret);
+    }
+  }
+
+  { // check that the comparator is used
+    {
+      int a[] = {5, 4, 3, 2};
+      auto ret = std::ranges::is_sorted(Iter(a), Sent(Iter(a + 4)), std::ranges::greater{});
+      assert(ret);
+    }
+    {
+      int a[] = {5, 4, 3, 2};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 4)));
+      auto ret = std::ranges::is_sorted(range, std::ranges::greater{});
+      assert(ret);
+    }
+  }
+
+  { // check that an empty range works
+    {
+      int a[] = {};
+      auto ret = std::ranges::is_sorted(Iter(a), Sent(Iter(a)));
+      assert(ret);
+    }
+    {
+      int a[] = {};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a)));
+      auto ret = std::ranges::is_sorted(range);
+      assert(ret);
+    }
+  }
+
+  { // check that a range with a single element works
+    {
+      int a[] = {32};
+      auto ret = std::ranges::is_sorted(Iter(a), Sent(Iter(a + 1)));
+      assert(ret);
+    }
+    {
+      int a[] = {32};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 1)));
+      auto ret = std::ranges::is_sorted(range);
+      assert(ret);
+    }
+  }
+}
+
+constexpr bool test() {
+  test_iterators<forward_iterator<int*>, sentinel_wrapper<forward_iterator<int*>>>();
+  test_iterators<forward_iterator<int*>>();
+  test_iterators<bidirectional_iterator<int*>>();
+  test_iterators<random_access_iterator<int*>>();
+  test_iterators<contiguous_iterator<int*>>();
+  test_iterators<int*>();
+  test_iterators<const int*>();
+
+  { // check that the projection is used
+    struct S {
+      int i;
+      constexpr S(int i_) : i(i_) {}
+    };
+    {
+      S a[] = {1, 2, 3};
+      auto ret = std::ranges::is_sorted(a, a + 3, {}, &S::i);
+      assert(ret);
+    }
+    {
+      S a[] = {1, 2, 3};
+      auto ret = std::ranges::is_sorted(a, {}, &S::i);
+      assert(ret);
+    }
+  }
+
+  { // check that a dangling range works
+    assert(std::ranges::is_sorted(std::array{1, 2, 3, 4}));
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/algorithms/alg.sorting/alg.sort/is.sorted/ranges.is_sorted_until.pass.cpp b/libcxx/test/std/algorithms/alg.sorting/alg.sort/is.sorted/ranges.is_sorted_until.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.sorting/alg.sort/is.sorted/ranges.is_sorted_until.pass.cpp
@@ -0,0 +1,180 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// <algorithm>
+
+// template<forward_iterator I, sentinel_for<I> S, class Proj = identity,
+//          indirect_strict_weak_order<projected<I, Proj>> Comp = ranges::less>
+//   constexpr I ranges::is_sorted_until(I first, S last, Comp comp = {}, Proj proj = {});
+// template<forward_range R, class Proj = identity,
+//          indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
+//   constexpr borrowed_iterator_t<R>
+//     ranges::is_sorted_until(R&& r, Comp comp = {}, Proj proj = {});
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <functional>
+#include <ranges>
+
+#include "almost_satisfies_types.h"
+#include "test_iterators.h"
+
+template <class Iter, class Sent = Iter>
+concept HasIsSortedUntilIt = requires(Iter iter, Sent sent) { std::ranges::is_sorted_until(iter, sent); };
+
+struct HasNoComparator {};
+
+static_assert(HasIsSortedUntilIt<int*>);
+static_assert(!HasIsSortedUntilIt<ForwardIteratorNotDerivedFrom>);
+static_assert(!HasIsSortedUntilIt<ForwardIteratorNotIncrementable>);
+static_assert(!HasIsSortedUntilIt<int*, SentinelForNotSemiregular>);
+static_assert(!HasIsSortedUntilIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
+static_assert(!HasIsSortedUntilIt<HasNoComparator*>);
+
+template <class Range>
+concept HasIsSortedUntilR = requires(Range range) { std::ranges::is_sorted_until(range); };
+
+static_assert(HasIsSortedUntilR<UncheckedRange<int*>>);
+static_assert(!HasIsSortedUntilR<ForwardRangeNotDerivedFrom>);
+static_assert(!HasIsSortedUntilR<ForwardRangeNotIncrementable>);
+static_assert(!HasIsSortedUntilR<ForwardRangeNotSentinelSemiregular>);
+static_assert(!HasIsSortedUntilR<ForwardRangeNotSentinelEqualityComparableWith>);
+static_assert(!HasIsSortedUntilR<UncheckedRange<HasNoComparator*>>);
+
+template <class Iter, class Sent = Iter>
+constexpr void test_iterators() {
+  { // simple test
+    {
+      int a[] = {1, 2, 3, 4, 3};
+      std::same_as<Iter> auto ret = std::ranges::is_sorted_until(Iter(a), Sent(Iter(a + 5)));
+      assert(base(ret) == a + 4);
+    }
+    {
+      int a[] = {1, 2, 3, 4, 3};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      std::same_as<Iter> auto ret = std::ranges::is_sorted_until(range);
+      assert(base(ret) == a + 4);
+    }
+  }
+
+  { // second element isn't sorted
+    {
+      int a[] = {1, 0, 3, 4, 5};
+      auto ret = std::ranges::is_sorted_until(Iter(a), Sent(Iter(a + 5)));
+      assert(base(ret) == a + 1);
+    }
+    {
+      int a[] = {1, 0, 3, 4, 5};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      auto ret = std::ranges::is_sorted_until(range);
+      assert(base(ret) == a + 1);
+    }
+  }
+
+  { // all elements are sorted
+    {
+      int a[] = {1, 2, 3, 4, 5};
+      auto ret = std::ranges::is_sorted_until(Iter(a), Sent(Iter(a + 5)));
+      assert(base(ret) == a + 5);
+    }
+    {
+      int a[] = {1, 2, 3, 4, 5};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 5)));
+      auto ret = std::ranges::is_sorted_until(range);
+      assert(base(ret) == a + 5);
+    }
+  }
+
+  { // check that the comparator is used
+    {
+      int a[] = {5, 4, 3, 2};
+      auto ret = std::ranges::is_sorted_until(Iter(a), Sent(Iter(a + 4)), std::ranges::greater{});
+      assert(base(ret) == a + 4);
+    }
+    {
+      int a[] = {5, 4, 3, 2};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 4)));
+      auto ret = std::ranges::is_sorted_until(range, std::ranges::greater{});
+      assert(base(ret) == a + 4);
+    }
+  }
+
+  { // check that an empty range works
+    {
+      int a[] = {};
+      auto ret = std::ranges::is_sorted_until(Iter(a), Sent(Iter(a)));
+      assert(base(ret) == a);
+    }
+    {
+      int a[] = {};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a)));
+      auto ret = std::ranges::is_sorted_until(range);
+      assert(base(ret) == a);
+    }
+  }
+
+  { // check that a range with a single element works
+    {
+      int a[] = {32};
+      auto ret = std::ranges::is_sorted_until(Iter(a), Sent(Iter(a + 1)));
+      assert(base(ret) == a + 1);
+    }
+    {
+      int a[] = {32};
+      auto range = std::ranges::subrange(Iter(a), Sent(Iter(a + 1)));
+      auto ret = std::ranges::is_sorted_until(range);
+      assert(base(ret) == a + 1);
+    }
+  }
+}
+
+constexpr bool test() {
+  test_iterators<forward_iterator<int*>, sentinel_wrapper<forward_iterator<int*>>>();
+  test_iterators<forward_iterator<int*>>();
+  test_iterators<bidirectional_iterator<int*>>();
+  test_iterators<random_access_iterator<int*>>();
+  test_iterators<contiguous_iterator<int*>>();
+  test_iterators<int*>();
+  test_iterators<const int*>();
+
+  { // check that the projection is used
+    struct S {
+      int i;
+      constexpr S(int i_) : i(i_) {}
+    };
+    {
+      S a[] = {1, 2, 3};
+      auto ret = std::ranges::is_sorted_until(a, a + 3, {}, &S::i);
+      assert(base(ret) == a + 3);
+    }
+    {
+      S a[] = {1, 2, 3};
+      auto ret = std::ranges::is_sorted_until(a, {}, &S::i);
+      assert(base(ret) == a + 3);
+    }
+  }
+
+  { // check that std::ranges::dangling is returned
+    [[maybe_unused]] std::same_as<std::ranges::dangling> decltype(auto) ret =
+        std::ranges::is_sorted_until(std::array{1, 2, 3});
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp b/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp
--- a/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp
+++ b/libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp
@@ -91,8 +91,8 @@
 //static_assert(test(std::ranges::is_heap_until, a));
 //static_assert(test(std::ranges::is_partitioned, a, odd));
 //static_assert(test(std::ranges::is_permutation, a, a));
-//static_assert(test(std::ranges::is_sorted, a));
-//static_assert(test(std::ranges::is_sorted_until, a));
+static_assert(test(std::ranges::is_sorted, a));
+static_assert(test(std::ranges::is_sorted_until, a));
 //static_assert(test(std::ranges::lexicographical_compare, a, a));
 //static_assert(test(std::ranges::lower_bound, a, 42));
 //static_assert(test(std::ranges::make_heap, a));
diff --git a/libcxx/test/support/almost_satisfies_types.h b/libcxx/test/support/almost_satisfies_types.h
--- a/libcxx/test/support/almost_satisfies_types.h
+++ b/libcxx/test/support/almost_satisfies_types.h
@@ -142,6 +142,49 @@
 static_assert(!std::movable<WeaklyIncrementableNotMovable>);
 static_assert(!std::weakly_incrementable<WeaklyIncrementableNotMovable>);
 
+// almost a forward_iterator
+class ForwardIteratorNotDerivedFrom {
+public:
+  using difference_type = long;
+  using value_type = int;
+  using iterator_category = std::input_iterator_tag;
+
+  ForwardIteratorNotDerivedFrom& operator++();
+  ForwardIteratorNotDerivedFrom operator++(int);
+  const int& operator*() const;
+  bool operator==(const ForwardIteratorNotDerivedFrom&) const = default;
+};
+
+using ForwardRangeNotDerivedFrom = UncheckedRange<ForwardIteratorNotDerivedFrom>;
+
+static_assert(std::input_iterator<ForwardIteratorNotDerivedFrom>);
+static_assert(std::incrementable<ForwardIteratorNotDerivedFrom>);
+static_assert(std::sentinel_for<ForwardIteratorNotDerivedFrom, ForwardIteratorNotDerivedFrom>);
+static_assert(!std::forward_iterator<ForwardIteratorNotDerivedFrom>);
+
+class ForwardIteratorNotIncrementable {
+public:
+  using difference_type = long;
+  using value_type = int;
+  using iterator_category = std::forward_iterator_tag;
+
+  ForwardIteratorNotIncrementable& operator++();
+  int operator++(int);
+  const int& operator*() const;
+  bool operator==(const ForwardIteratorNotIncrementable&) const = default;
+};
+
+using ForwardRangeNotIncrementable = UncheckedRange<ForwardIteratorNotIncrementable>;
+
+static_assert(std::input_iterator<ForwardIteratorNotIncrementable>);
+static_assert(!std::incrementable<ForwardIteratorNotIncrementable>);
+static_assert(std::sentinel_for<ForwardIteratorNotIncrementable, ForwardIteratorNotIncrementable>);
+static_assert(!std::forward_iterator<ForwardIteratorNotIncrementable>);
+
+using ForwardRangeNotSentinelSemiregular = UncheckedRange<forward_iterator<int*>, SentinelForNotSemiregular>;
+using ForwardRangeNotSentinelEqualityComparableWith =
+    UncheckedRange<forward_iterator<int*>, SentinelForNotWeaklyEqualityComparableWith>;
+
 class BidirectionalIteratorNotDerivedFrom {
 public:
   using difference_type = long;