diff --git a/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp b/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp
--- a/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp
+++ b/libcxx/test/std/numerics/bit/bit.cast/bit_cast.pass.cpp
@@ -21,6 +21,8 @@
 #include <cstring>
 #include <limits>
 
+#include "test_macros.h"
+
 // std::bit_cast does not preserve padding bits, so if T has padding bits,
 // the results might not memcmp cleanly.
 template<bool HasUniqueObjectRepresentations = true, typename T>
@@ -230,7 +232,8 @@
 #if __SIZEOF_LONG_DOUBLE__ == __SIZEOF_DOUBLE__
         test_roundtrip_through<double, false>(i);
 #endif
-#if defined(__SIZEOF_INT128__) && __SIZEOF_LONG_DOUBLE__ == __SIZEOF_INT128__
+#if defined(__SIZEOF_INT128__) && __SIZEOF_LONG_DOUBLE__ == __SIZEOF_INT128__ &&                                       \
+    !TEST_HAS_FEATURE(memory_sanitizer) // Some bits are just padding.
         test_roundtrip_through<__int128_t, false>(i);
         test_roundtrip_through<__uint128_t, false>(i);
 #endif