diff --git a/llvm/lib/Support/BLAKE3/blake3_impl.h b/llvm/lib/Support/BLAKE3/blake3_impl.h
--- a/llvm/lib/Support/BLAKE3/blake3_impl.h
+++ b/llvm/lib/Support/BLAKE3/blake3_impl.h
@@ -54,8 +54,9 @@
 #endif
 
 #if !defined(BLAKE3_USE_NEON) 
-  // If BLAKE3_USE_NEON not manually set, autodetect based on AArch64ness
-  #if defined(IS_AARCH64)
+  // If BLAKE3_USE_NEON not manually set, autodetect based on
+  // AArch64ness and endianness.
+  #if defined(IS_AARCH64) && !defined(__ARM_BIG_ENDIAN)
     #define BLAKE3_USE_NEON 1
   #else
     #define BLAKE3_USE_NEON 0