Index: libcxx/include/unordered_map
===================================================================
--- libcxx/include/unordered_map
+++ libcxx/include/unordered_map
@@ -183,6 +183,8 @@
     mapped_type&       at(const key_type& k);
     const mapped_type& at(const key_type& k) const;
 
+    bool contains(const key_type& k) const; // C++20
+
     size_type bucket_count() const noexcept;
     size_type max_bucket_count() const noexcept;
 
@@ -358,6 +360,8 @@
     pair<iterator, iterator>             equal_range(const key_type& k);
     pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
 
+    bool contains(const key_type& k) const; // C++20
+
     size_type bucket_count() const noexcept;
     size_type max_bucket_count() const noexcept;
 
@@ -1290,6 +1294,11 @@
     mapped_type&       at(const key_type& __k);
     const mapped_type& at(const key_type& __k) const;
 
+#if _LIBCPP_STD_VER > 17
+    _LIBCPP_INLINE_VISIBILITY
+    bool contains(const key_type& __k) const { return  __table_.find(__k) !=  __table_.end(); }
+#endif
+
     _LIBCPP_INLINE_VISIBILITY
     size_type bucket_count() const _NOEXCEPT {return __table_.bucket_count();}
     _LIBCPP_INLINE_VISIBILITY
@@ -1983,6 +1992,12 @@
     pair<const_iterator, const_iterator> equal_range(const key_type& __k) const
         {return __table_.__equal_range_multi(__k);}
 
+#if _LIBCPP_STD_VER > 17
+    _LIBCPP_INLINE_VISIBILITY
+    bool contains(const key_type& __k) const
+        { return  __table_.find(__k) !=  __table_.end(); }
+#endif
+
     _LIBCPP_INLINE_VISIBILITY
     size_type bucket_count() const _NOEXCEPT {return __table_.bucket_count();}
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/include/unordered_set
===================================================================
--- libcxx/include/unordered_set
+++ libcxx/include/unordered_set
@@ -149,6 +149,8 @@
     pair<iterator, iterator>             equal_range(const key_type& k);
     pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
 
+    bool contains(const key_type& k) const; // C++20
+
     size_type bucket_count() const noexcept;
     size_type max_bucket_count() const noexcept;
 
@@ -313,6 +315,8 @@
     pair<iterator, iterator>             equal_range(const key_type& k);
     pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
 
+    bool contains(const key_type& k) const; // C++20
+
     size_type bucket_count() const noexcept;
     size_type max_bucket_count() const noexcept;
 
@@ -681,6 +685,12 @@
     pair<const_iterator, const_iterator> equal_range(const key_type& __k) const
         {return __table_.__equal_range_unique(__k);}
 
+#if _LIBCPP_STD_VER > 17
+    _LIBCPP_INLINE_VISIBILITY
+    bool contains(const key_type& __k) const
+        { return __table_.find(__k) != __table_.end(); }
+#endif
+
     _LIBCPP_INLINE_VISIBILITY
     size_type bucket_count() const _NOEXCEPT {return __table_.bucket_count();}
     _LIBCPP_INLINE_VISIBILITY
@@ -1249,6 +1259,12 @@
     pair<const_iterator, const_iterator> equal_range(const key_type& __k) const
         {return __table_.__equal_range_multi(__k);}
 
+#if _LIBCPP_STD_VER > 17
+    _LIBCPP_INLINE_VISIBILITY
+    bool contains(const key_type& __k) const
+        { return __table_.find(__k) != __table_.end(); }
+#endif
+
     _LIBCPP_INLINE_VISIBILITY
     size_type bucket_count() const _NOEXCEPT {return __table_.bucket_count();}
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/test/std/containers/unord/unord.map/contains.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.map/contains.pass.cpp
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++98, c++03, c++11, c++14, c++17
+// <unordered_map>
+
+// template <class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>,
+//           class Alloc = allocator<pair<const Key, T>>>
+// class unordered_map
+
+// bool contains(const key_type& k) const;
+
+#include <unordered_map>
+#include <string>
+#include <cassert>
+
+int main(int, char**)
+{
+    std::unordered_map<int, std::string> c{};
+    assert(!c.contains(1));
+
+    c[1] = "one";
+    assert(c.contains(1));
+
+    return 0;
+}
Index: libcxx/test/std/containers/unord/unord.multimap/contains.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.multimap/contains.pass.cpp
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++98, c++03, c++11, c++14, c++17
+// <unordered_map>
+
+// template <class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>,
+//           class Alloc = allocator<pair<const Key, T>>>
+// class unordered_multimap
+
+// bool contains(const key_type& k) const;
+
+#include <unordered_map>
+#include <string>
+#include <cassert>
+
+int main(int, char**)
+{
+    std::unordered_multimap<int, std::string> c{};
+    assert(!c.contains(1));
+
+    c.insert({1, "one"});
+    assert(c.contains(1));
+
+    return 0;
+}
Index: libcxx/test/std/containers/unord/unord.multiset/contains.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.multiset/contains.pass.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++98, c++03, c++11, c++14, c++17
+// <unordered_set>
+
+// template <class Value, class Hash = hash<Value>, class Pred = equal_to<Value>,
+//           class Alloc = allocator<Value>>
+// class unordered_multiset
+
+// bool contains(const key_type& k) const;
+
+#include <unordered_set>
+#include <cassert>
+
+int main(int, char**)
+{
+    std::unordered_multiset<int> c{};
+    assert(!c.contains(1));
+
+    c.insert(1);
+    assert(c.contains(1));
+
+    return 0;
+}
Index: libcxx/test/std/containers/unord/unord.set/contains.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.set/contains.pass.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++98, c++03, c++11, c++14, c++17
+// <unordered_set>
+
+// template <class Value, class Hash = hash<Value>, class Pred = equal_to<Value>,
+//           class Alloc = allocator<Value>>
+// class unordered_set
+
+// bool contains(const key_type& k) const;
+
+#include <unordered_set>
+#include <cassert>
+
+int main(int, char**)
+{
+    std::unordered_set<int> c{};
+    assert(!c.contains(1));
+
+    c.insert(1);
+    assert(c.contains(1));
+
+    return 0;
+}