diff --git a/CMakeLists.txt b/CMakeLists.txt
index a7c8bedf..d12a2423 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -280,6 +280,14 @@ target_compile_definitions(${PROJECT_NAME}
         $<$<C_COMPILER_ID:MSVC>:_CRT_SECURE_NO_WARNINGS>
 )
 
+if(CMAKE_C_COMPILER_ID STREQUAL "Clang" AND CMAKE_C_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
+    target_compile_options(${PROJECT_NAME}
+        PRIVATE
+            # aead_aes256gcm_aesni requires -mssse3 when compiling w/ clang-cl.
+            -mssse3
+    )
+endif()
+
 # Variables that need to be exported to version.h.in
 set(VERSION 1.0.18)
 set(SODIUM_LIBRARY_VERSION_MAJOR 10)