diff --git a/stl/inc/__msvc_all_public_headers.hpp b/stl/inc/__msvc_all_public_headers.hpp
index b057b74fcbe..3d7a53d4724 100644
--- a/stl/inc/__msvc_all_public_headers.hpp
+++ b/stl/inc/__msvc_all_public_headers.hpp
@@ -35,6 +35,7 @@
 #endif // TRANSITION, OS-17090155 (UCRT)
 
 #define _SILENCE_CXX17_C_HEADER_DEPRECATION_WARNING
+#define _SILENCE_CXX20_CISO646_REMOVED_WARNING
 #define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING
 #define _SILENCE_STDEXT_HASH_DEPRECATION_WARNINGS
 
diff --git a/stl/inc/ciso646 b/stl/inc/ciso646
index 31615ef032f..02c3e72441a 100644
--- a/stl/inc/ciso646
+++ b/stl/inc/ciso646
@@ -11,5 +11,8 @@
 
 #include <iso646.h>
 
+_CXX20_REMOVE_CISO646 typedef int _Header_ciso646;
+using _Hdr_ciso646 = _Header_ciso646;
+
 #endif // _STL_COMPILER_PREPROCESSOR
 #endif // _CISO646_
diff --git a/stl/inc/yvals_core.h b/stl/inc/yvals_core.h
index 89fa6384562..7efe7546ab5 100644
--- a/stl/inc/yvals_core.h
+++ b/stl/inc/yvals_core.h
@@ -1171,7 +1171,17 @@
 #define _CXX23_DEPRECATE_ALIGNED_UNION
 #endif // ^^^ warning disabled ^^^
 
-// next warning number: STL4036
+#if _HAS_CXX20 && !defined(_SILENCE_CXX20_CISO646_REMOVED_WARNING) && !defined(_SILENCE_ALL_CXX20_DEPRECATION_WARNINGS)
+#define _CXX20_REMOVE_CISO646                                             \
+    [[deprecated("warning STL4036: "                                      \
+                 "<ciso646> is removed in C++20. "                        \
+                 "You can define _SILENCE_CXX20_CISO646_REMOVED_WARNING " \
+                 "or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to acknowledge that you have received this warning.")]]
+#else // ^^^ warning enabled / warning disabled vvv
+#define _CXX20_REMOVE_CISO646
+#endif // ^^^ warning disabled ^^^
+
+// next warning number: STL4037
 
 // P0619R4 Removing C++17-Deprecated Features
 #ifndef _HAS_FEATURES_REMOVED_IN_CXX20
diff --git a/tests/std/tests/P0088R3_variant/test.cpp b/tests/std/tests/P0088R3_variant/test.cpp
index 836a6edb920..278a47632fb 100644
--- a/tests/std/tests/P0088R3_variant/test.cpp
+++ b/tests/std/tests/P0088R3_variant/test.cpp
@@ -37,6 +37,7 @@
 
 #define _HAS_DEPRECATED_RESULT_OF 1
 #define _SILENCE_CXX17_RESULT_OF_DEPRECATION_WARNING
+#define _SILENCE_CXX20_CISO646_REMOVED_WARNING
 #define _SILENCE_CXX20_VOLATILE_DEPRECATION_WARNING
 #define _SILENCE_CXX23_ALIGNED_UNION_DEPRECATION_WARNING
 #define _LIBCXX_IN_DEVCRT
diff --git a/tests/std/tests/P0220R1_any/test.cpp b/tests/std/tests/P0220R1_any/test.cpp
index 21545e4eed6..a13fe517d10 100644
--- a/tests/std/tests/P0220R1_any/test.cpp
+++ b/tests/std/tests/P0220R1_any/test.cpp
@@ -31,6 +31,7 @@
 
 #define _HAS_DEPRECATED_IS_LITERAL_TYPE 1
 #define _SILENCE_CXX17_IS_LITERAL_TYPE_DEPRECATION_WARNING
+#define _SILENCE_CXX20_CISO646_REMOVED_WARNING
 #define _LIBCXX_IN_DEVCRT
 #include <msvc_stdlib_force_include.h> // Must precede any other libc++ headers
 #include <stdlib.h>
diff --git a/tests/std/tests/P0220R1_optional/test.cpp b/tests/std/tests/P0220R1_optional/test.cpp
index 648b147c6fd..bbd353ca3b2 100644
--- a/tests/std/tests/P0220R1_optional/test.cpp
+++ b/tests/std/tests/P0220R1_optional/test.cpp
@@ -34,6 +34,7 @@
 #define _HAS_DEPRECATED_RESULT_OF       1
 #define _SILENCE_CXX17_IS_LITERAL_TYPE_DEPRECATION_WARNING
 #define _SILENCE_CXX17_RESULT_OF_DEPRECATION_WARNING
+#define _SILENCE_CXX20_CISO646_REMOVED_WARNING
 #define _LIBCXX_IN_DEVCRT
 #include <msvc_stdlib_force_include.h> // Must precede any other libc++ headers
 #include <stdlib.h>
diff --git a/tests/std/tests/VSO_0000000_instantiate_iterators_misc/test.compile.pass.cpp b/tests/std/tests/VSO_0000000_instantiate_iterators_misc/test.compile.pass.cpp
index 10f7e9a686f..6a0e87e6a60 100644
--- a/tests/std/tests/VSO_0000000_instantiate_iterators_misc/test.compile.pass.cpp
+++ b/tests/std/tests/VSO_0000000_instantiate_iterators_misc/test.compile.pass.cpp
@@ -32,7 +32,6 @@
 #include <cfloat>
 #include <chrono>
 #include <cinttypes>
-#include <ciso646>
 #include <climits>
 #include <clocale>
 #include <cmath>
diff --git a/tests/std/tests/include_each_header_alone_matrix.lst b/tests/std/tests/include_each_header_alone_matrix.lst
index 731434c1507..b6a08effe83 100644
--- a/tests/std/tests/include_each_header_alone_matrix.lst
+++ b/tests/std/tests/include_each_header_alone_matrix.lst
@@ -102,7 +102,7 @@ PM_CL="/DMEOW_HEADER=cerrno"
 PM_CL="/DMEOW_HEADER=cfenv"
 PM_CL="/DMEOW_HEADER=cfloat"
 PM_CL="/DMEOW_HEADER=cinttypes"
-PM_CL="/DMEOW_HEADER=ciso646"
+PM_CL="/DMEOW_HEADER=ciso646 /D_SILENCE_CXX20_CISO646_REMOVED_WARNING"
 PM_CL="/DMEOW_HEADER=climits"
 PM_CL="/DMEOW_HEADER=clocale"
 PM_CL="/DMEOW_HEADER=cmath"
diff --git a/tests/tr1/tests/ciso646/test.cpp b/tests/tr1/tests/ciso646/test.cpp
index e1bcce02988..a53f8b2dc66 100644
--- a/tests/tr1/tests/ciso646/test.cpp
+++ b/tests/tr1/tests/ciso646/test.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 // test <ciso646>
+#define _SILENCE_CXX20_CISO646_REMOVED_WARNING
 #define TEST_NAME "<ciso646>"
 
 #include "tdefs.h"