Index: libcxx/include/memory
===================================================================
--- libcxx/include/memory
+++ libcxx/include/memory
@@ -953,6 +953,19 @@
   }
 };
 
+#if _LIBCPP_STD_VER >= 20
+template <size_t N, typename T>
+[[nodiscard]]
+_LIBCPP_CONSTEXPR
+_LIBCPP_ALWAYS_INLINE
+T* assume_aligned(T* __ptr)
+#if defined(__clang__) ||  defined(__GNUC__)
+  __attribute__((assume_aligned(N)))
+#endif
+{
+    return __ptr;
+}
+#endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
 
Index: libcxx/test/libcxx/memory/assume_aligned.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/memory/assume_aligned.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
+//
+//===----------------------------------------------------------------------===//
+//
+
+// <memory>
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// template<std::size_t N, typename T>
+// [[nodiscard]] T* assume_aligned(T*);
+
+#include <memory>
+
+constexpr int test() {
+  int a;
+  [[maybe_unused]] int* b = std::assume_aligned<4>(&a);
+  return sizeof(int);
+}
+
+int main(int, char**)
+{
+  constexpr int N = test();
+  int* a = (int*)malloc(N);
+  [[maybe_unused]] int* b = std::assume_aligned<4>(a);
+
+  return 0;
+}