diff --git a/libcxx/include/__config b/libcxx/include/__config
index 77a71b6cf1cae..2ce0c4448aae9 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1021,14 +1021,27 @@ typedef __char32_t char32_t;
 // the latter depends on internal GNU libc details that are not appropriate
 // to depend on here, so any declarations present when __cpp_char8_t is not
 // defined are ignored.
+//
+// picolibc 1.8.9 and newer declare the two functions unconditionally.
+//
+// Apple C library, Microsoft C Runtime, AIX C library and FreeBSD libc do not
+// yet support these functions.
 #  if defined(_LIBCPP_GLIBC_PREREQ)
-#    if _LIBCPP_GLIBC_PREREQ(2, 36) && defined(__cpp_char8_t)
+#    if defined(__cpp_char8_t) && _LIBCPP_GLIBC_PREREQ(2, 36)
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
 #    else
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
 #    endif
-#  else
+#  elif defined(_LIBCPP_PICOLIBC_PREREQ)
+#    if _LIBCPP_PICOLIBC_PREREQ(1, 8, 9)
+#      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
+#    else
+#      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
+#    endif
+#  elif defined(__APPLE__) || defined(_MSC_VER) || defined(_LIBCPP_UCRT) || defined(_AIX) || defined(__FreeBSD__)
 #    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
+#  else
+#    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
 #  endif
 
 // There are a handful of public standard library types that are intended to
diff --git a/libcxx/include/__configuration/platform.h b/libcxx/include/__configuration/platform.h
index f3c199dee172b..a448ca82b2935 100644
--- a/libcxx/include/__configuration/platform.h
+++ b/libcxx/include/__configuration/platform.h
@@ -47,6 +47,17 @@
 //       user code. Move code paths that need _NEWLIB_VERSION to another customization mechanism.
 #if __has_include(<picolibc.h>)
 #  include <picolibc.h>
+#  define _LIBCPP_PICOLIBC_VERSION_INT(maj, min, patch) (maj * 10000 + min * 100 + patch)
+#  define _LIBCPP_PICOLIBC_PREREQ(maj, min, patch)                                                                     \
+    _LIBCPP_PICOLIBC_VERSION_INT(__PICOLIBC__, __PICOLIBC_MINOR__, __PICOLIBC_PATCHLEVEL__) >=                         \
+        _LIBCPP_PICOLIBC_VERSION_INT(maj, min, patch)
+#endif
+
+#if __has_include(<corecrt.h>)
+#  include <corecrt.h>
+#  if defined(_UCRT)
+#    define _LIBCPP_UCRT
+#  endif
 #endif
 
 #ifndef __BYTE_ORDER__