diff --git a/src/Reactor/BUILD.gn b/src/Reactor/BUILD.gn
index 676f99f..86f4bc6 100644
--- a/src/Reactor/BUILD.gn
+++ b/src/Reactor/BUILD.gn
@@ -27,6 +27,11 @@
     ]
   } else {
     cflags = [ "-Wno-unused-local-typedef" ]
+    defines = [
+      # Enable instrumentation of Reactor routines for MemorySanitizer builds (LLVM backend).
+      # TODO(b/155148722): Remove when unconditionally instrumenting for all build systems.
+      "REACTOR_ENABLE_MEMORY_SANITIZER_INSTRUMENTATION=1",
+    ]
   }
 }