diff --git a/src/dsp/cpu.h b/src/dsp/cpu.h
index 40711b2f..be0c280f 100644
--- a/src/dsp/cpu.h
+++ b/src/dsp/cpu.h
@@ -147,6 +147,12 @@
 #endif
 #endif
 
+#if defined(__has_feature)
+#if __has_feature(memory_sanitizer)
+#define WEBP_MSAN
+#endif
+#endif
+
 #if defined(WEBP_USE_THREAD) && !defined(_WIN32)
 #include <pthread.h>  // NOLINT
 
diff --git a/src/enc/picture_csp_enc.c b/src/enc/picture_csp_enc.c
index 5e60f5ba..fabebcf2 100644
--- a/src/enc/picture_csp_enc.c
+++ b/src/enc/picture_csp_enc.c
@@ -451,11 +451,20 @@ static WEBP_INLINE void AccumulateRGB(const uint8_t* const r_ptr,
     dst[0] = SUM4(r_ptr + j, step);
     dst[1] = SUM4(g_ptr + j, step);
     dst[2] = SUM4(b_ptr + j, step);
+    // MemorySanitizer may raise false positives with data that passes through
+    // RGBA32PackedToPlanar_16b_SSE41() due to incorrect modeling of shuffles.
+    // See https://crbug.com/webp/573.
+#ifdef WEBP_MSAN
+    dst[3] = 0;
+#endif
   }
   if (width & 1) {
     dst[0] = SUM2(r_ptr + j);
     dst[1] = SUM2(g_ptr + j);
     dst[2] = SUM2(b_ptr + j);
+#ifdef WEBP_MSAN
+    dst[3] = 0;
+#endif
   }
 }