diff --git a/package.json b/package.json
index 3f3b0515d..141aec741 100644
--- a/package.json
+++ b/package.json
@@ -14,7 +14,7 @@
 		"node": ">=12.20"
 	},
 	"scripts": {
-		"test": "xo && tsd && tsc"
+		"test": "xo && tsd && tsc && node script/test/source-files-extension.js"
 	},
 	"files": [
 		"index.d.ts",
diff --git a/script/test/source-files-extension.js b/script/test/source-files-extension.js
new file mode 100755
index 000000000..b29664be6
--- /dev/null
+++ b/script/test/source-files-extension.js
@@ -0,0 +1,26 @@
+/* eslint-disable unicorn/prefer-module */
+const fs = require('fs');
+const process = require('process');
+
+const checkSourceFilesExtension = async () => {
+	try {
+		const files = await fs.promises.readdir('source');
+
+		let hasIncorrectFileExtension = false;
+		for (const file of files) {
+			if (!file.endsWith('.d.ts')) {
+				hasIncorrectFileExtension = true;
+				console.error(`source/${file} extension should be \`.d.ts\`.`);
+			}
+		}
+
+		if (hasIncorrectFileExtension) {
+			process.exitCode = 1;
+		}
+	} catch (error) {
+		console.error(error);
+		process.exitCode = 1;
+	}
+};
+
+checkSourceFilesExtension();