diff --git a/dom/workers/RuntimeService.cpp b/dom/workers/RuntimeService.cpp
index 78ffcb8728b1..6211d5818939 100644
--- a/dom/workers/RuntimeService.cpp
+++ b/dom/workers/RuntimeService.cpp
@@ -752,6 +752,18 @@ InitJSContextForWorker(WorkerPrivate* aWorkerPrivate, JSContext* aWorkerCx)
   JSSettings settings;
   aWorkerPrivate->CopyJSSettings(settings);
 
+  {
+    JS::UniqueChars defaultLocale = aWorkerPrivate->AdoptDefaultLocale();
+    MOZ_ASSERT(defaultLocale,
+               "failure of a WorkerPrivate to have a default locale should "
+               "have made the worker fail to spawn");
+
+    if (!JS_SetDefaultLocale(aWorkerCx, defaultLocale.get())) {
+      NS_WARNING("failed to set workerCx's default locale");
+      return nullptr;
+    }
+  }
+
   JS::ContextOptionsRef(aWorkerCx) = settings.contextOptions;
 
   JSSettings::JSGCSettingsArray& gcSettings = settings.gcSettings;
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index f61c803602d0..a7faafa21094 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -42,6 +42,7 @@
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/Likely.h"
 #include "mozilla/LoadContext.h"
+#include "mozilla/Move.h"
 #include "mozilla/dom/BindingUtils.h"
 #include "mozilla/dom/Console.h"
 #include "mozilla/dom/ErrorEvent.h"
@@ -4196,6 +4197,18 @@ WorkerPrivate::Constructor(JSContext* aCx,
     new WorkerPrivate(parent, aScriptURL, aIsChromeWorker,
                       aWorkerType, aWorkerName, *aLoadInfo);
 
+  // Gecko contexts always have an explicitly-set default locale (set by
+  // XPJSRuntime::Initialize for the main thread, set by
+  // WorkerThreadPrimaryRunnable::Run for workers just before running worker
+  // code), so this is never SpiderMonkey's builtin default locale.
+  JS::UniqueChars defaultLocale = JS_GetDefaultLocale(aCx);
+  if (NS_WARN_IF(!defaultLocale)) {
+    aRv.Throw(NS_ERROR_UNEXPECTED);
+    return nullptr;
+  }
+
+  worker->mDefaultLocale = Move(defaultLocale);
+
   if (!runtimeService->RegisterWorker(worker)) {
     aRv.Throw(NS_ERROR_UNEXPECTED);
     return nullptr;
diff --git a/dom/workers/WorkerPrivate.h b/dom/workers/WorkerPrivate.h
index 80209fd6ad11..82bec54d604f 100644
--- a/dom/workers/WorkerPrivate.h
+++ b/dom/workers/WorkerPrivate.h
@@ -15,8 +15,11 @@
 #include "nsIWorkerDebugger.h"
 #include "nsPIDOMWindow.h"
 
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
 #include "mozilla/CondVar.h"
 #include "mozilla/DOMEventTargetHelper.h"
+#include "mozilla/Move.h"
 #include "mozilla/TimeStamp.h"
 #include "mozilla/dom/BindingDeclarations.h"
 #include "nsAutoPtr.h"
@@ -938,6 +941,7 @@ class WorkerPrivate : public WorkerPrivateParent<WorkerPrivate>
   // fired on the main thread if the worker script fails to load
   nsCOMPtr<nsIRunnable> mLoadFailedRunnable;
 
+  JS::UniqueChars mDefaultLocale; // nulled during worker JSContext init
   TimeStamp mKillTime;
   uint32_t mErrorHandlerRecursionCount;
   uint32_t mNextTimeoutId;
@@ -1049,6 +1053,15 @@ public:
     mDebugger = aDebugger;
   }
 
+  JS::UniqueChars
+  AdoptDefaultLocale()
+  {
+    MOZ_ASSERT(mDefaultLocale,
+               "the default locale must have been successfully set for anyone "
+               "to be trying to adopt it");
+    return Move(mDefaultLocale);
+  }
+
   void
   DoRunLoop(JSContext* aCx);
 
diff --git a/intl/locale/tests/unit/data/chrome.manifest b/intl/locale/tests/unit/data/chrome.manifest
new file mode 100644
index 000000000000..a8678deef398
--- /dev/null
+++ b/intl/locale/tests/unit/data/chrome.manifest
@@ -0,0 +1 @@
+content locale ./
diff --git a/intl/locale/tests/unit/data/intl_on_workers_worker.js b/intl/locale/tests/unit/data/intl_on_workers_worker.js
new file mode 100644
index 000000000000..98aeeb5c0b29
--- /dev/null
+++ b/intl/locale/tests/unit/data/intl_on_workers_worker.js
@@ -0,0 +1,4 @@
+self.onmessage = function (data) {
+  let myLocale = Intl.NumberFormat().resolvedOptions().locale;
+  self.postMessage(myLocale);
+};
diff --git a/intl/locale/tests/unit/test_intl_on_workers.js b/intl/locale/tests/unit/test_intl_on_workers.js
new file mode 100644
index 000000000000..899e39d389da
--- /dev/null
+++ b/intl/locale/tests/unit/test_intl_on_workers.js
@@ -0,0 +1,23 @@
+function run_test() {
+  do_load_manifest("data/chrome.manifest");
+
+  if (typeof Intl !== "object") {
+    dump("Intl not enabled, skipping test\n");
+    equal(true, true);
+    return;
+  }
+
+  let mainThreadLocale = Intl.NumberFormat().resolvedOptions().locale;
+  let testWorker = new Worker("chrome://locale/content/intl_on_workers_worker.js");
+  testWorker.onmessage = function (e) {
+    try {
+      let workerLocale = e.data;
+      equal(mainThreadLocale, workerLocale, "Worker should inherit Intl locale from main thread.");
+    } finally {
+      do_test_finished();
+    }
+  };
+
+  do_test_pending();
+  testWorker.postMessage("go!");
+}
diff --git a/intl/locale/tests/unit/xpcshell.ini b/intl/locale/tests/unit/xpcshell.ini
index 88302be5e999..bde7384d2fb4 100644
--- a/intl/locale/tests/unit/xpcshell.ini
+++ b/intl/locale/tests/unit/xpcshell.ini
@@ -2,6 +2,9 @@
 head = 
 tail = 
 skip-if = toolkit == 'gonk'
+support-files =
+  data/intl_on_workers_worker.js
+  data/chrome.manifest
 
 [test_bug22310.js]
 skip-if = toolkit != "windows" && toolkit != "cocoa"
@@ -14,6 +17,7 @@ skip-if = toolkit == "windows" || toolkit == "cocoa"
 skip-if = toolkit != "cocoa"
 
 [test_bug1086527.js]
+[test_intl_on_workers.js]
 [test_pluralForm.js]
 [test_pluralForm_english.js]
 [test_pluralForm_makeGetter.js]
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index eabc1c869b23..678833d331e2 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -5856,6 +5856,16 @@ JS_SetDefaultLocale(JSContext* cx, const char* locale)
     return cx->setDefaultLocale(locale);
 }
 
+JS_PUBLIC_API(UniqueChars)
+JS_GetDefaultLocale(JSContext* cx)
+{
+    AssertHeapIsIdle(cx);
+    if (const char* locale = cx->getDefaultLocale())
+        return UniqueChars(JS_strdup(cx, locale));
+
+    return nullptr;
+}
+
 JS_PUBLIC_API(void)
 JS_ResetDefaultLocale(JSContext* cx)
 {
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 90f940bc4c1d..140b6e4ec32c 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -5060,6 +5060,12 @@ JS_ParseJSONWithReviver(JSContext* cx, JS::HandleString str, JS::HandleValue rev
 extern JS_PUBLIC_API(bool)
 JS_SetDefaultLocale(JSContext* cx, const char* locale);
 
+/**
+ * Look up the default locale for the ECMAScript Internationalization API.
+ */
+extern JS_PUBLIC_API(JS::UniqueChars)
+JS_GetDefaultLocale(JSContext* cx);
+
 /**
  * Reset the default locale to OS defaults.
  */