diff --git a/llvm/lib/Support/BLAKE3/CMakeLists.txt b/llvm/lib/Support/BLAKE3/CMakeLists.txt
--- a/llvm/lib/Support/BLAKE3/CMakeLists.txt
+++ b/llvm/lib/Support/BLAKE3/CMakeLists.txt
@@ -43,7 +43,7 @@
       blake3_avx2_x86-64_windows_msvc.asm
       blake3_avx512_x86-64_windows_msvc.asm
     )
-  elseif(WIN32)
+  elseif(WIN32 OR CYGWIN)
     list(APPEND LLVM_BLAKE3_FILES
       blake3_sse2_x86-64_windows_gnu.S
       blake3_sse41_x86-64_windows_gnu.S