diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 000000000000..dcbbceff83ce
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,7 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# This tells Github to detect files having the extension `.def` as `C++` files, which
+# ensures that these files get syntax highlighted properly.
+*.def linguist-language=C++