diff --git a/testing/mod.ts b/testing/mod.ts
index d1ddb9c8ce7b..51fa62cea60f 100644
--- a/testing/mod.ts
+++ b/testing/mod.ts
@@ -69,8 +69,29 @@ function print(txt: string, newline: boolean = true): void {
   Deno.stdout.writeSync(encoder.encode(`${txt}`));
 }
 
+declare global {
+  interface Window {
+    /**
+     * A global property to collect all registered test cases.
+     *
+     * It is required because user's code can import multiple versions
+     * of `testing` module.
+     *
+     * If test cases aren't registered in a globally shared
+     * object, then imports from different versions would register test cases
+     * to registry from it's respective version of `testing` module.
+     */
+    __DENO_TEST_REGISTRY: TestDefinition[];
+  }
+}
+
+let candidates: TestDefinition[] = [];
+if (window["__DENO_TEST_REGISTRY"]) {
+  candidates = window.__DENO_TEST_REGISTRY as TestDefinition[];
+} else {
+  window["__DENO_TEST_REGISTRY"] = candidates;
+}
 let filterRegExp: RegExp | null;
-const candidates: TestDefinition[] = [];
 let filtered = 0;
 
 // Must be called before any test() that needs to be filtered.