diff --git a/docs/_writing-tests/testharness-api.md b/docs/_writing-tests/testharness-api.md
index d750086c6a3a76..baa8772b814656 100644
--- a/docs/_writing-tests/testharness-api.md
+++ b/docs/_writing-tests/testharness-api.md
@@ -668,6 +668,17 @@ or a [`ServiceWorker`](https://slightlyoff.github.io/ServiceWorker/spec/service_
 Once called, the containing document fetches all the tests from the worker and
 behaves as if those tests were running in the containing document itself.
 
+`fetch_tests_from_worker` returns a promise that resolves once all the remote
+tests have completed. This is useful if you're importing tests from multiple
+workers and want to ensure they run in series:
+
+```js
+(async function() {
+  await fetch_tests_from_worker(new Worker("worker-1.js"));
+  await fetch_tests_from_worker(new Worker("worker-2.js"));
+})();
+```
+
 ## List of Assertions ##
 
 ### `assert_true(actual, description)`
diff --git a/resources/testharness.js b/resources/testharness.js
index 29dc6784459f6c..a48a0cfd3ec001 100644
--- a/resources/testharness.js
+++ b/resources/testharness.js
@@ -1780,6 +1780,12 @@ policies and contribution forms [3].
             }
         };
 
+        if (self.Promise) {
+            this.done = new Promise(function(resolve) {
+                this_obj.doneResolve = resolve;
+            });
+        }
+
         this.message_target.addEventListener("message", this.message_handler);
     }
 
@@ -1829,6 +1835,10 @@ policies and contribution forms [3].
         this.running = false;
         this.remote = null;
         this.message_target = null;
+        if (this.doneResolve) {
+            this.doneResolve();
+        }
+
         if (tests.all_done()) {
             tests.complete();
         }
@@ -2187,11 +2197,13 @@ policies and contribution forms [3].
             return;
         }
 
-        this.pending_remotes.push(this.create_remote_worker(worker));
+        var remoteContext = this.create_remote_worker(worker);
+        this.pending_remotes.push(remoteContext);
+        return remoteContext.done;
     };
 
     function fetch_tests_from_worker(port) {
-        tests.fetch_tests_from_worker(port);
+        return tests.fetch_tests_from_worker(port);
     }
     expose(fetch_tests_from_worker, 'fetch_tests_from_worker');