diff --git a/dom/payments/PaymentRequestManager.cpp b/dom/payments/PaymentRequestManager.cpp
index 0bd26fbbc1a7..05f3f1077bba 100644
--- a/dom/payments/PaymentRequestManager.cpp
+++ b/dom/payments/PaymentRequestManager.cpp
@@ -478,13 +478,18 @@ PaymentRequestManager::AbortPayment(PaymentRequest* aRequest, bool aDeferredShow
 
 nsresult
 PaymentRequestManager::CompletePayment(PaymentRequest* aRequest,
-                                       const PaymentComplete& aComplete)
+                                       const PaymentComplete& aComplete,
+                                       bool aTimedOut)
 {
   nsString completeStatusString(NS_LITERAL_STRING("unknown"));
-  uint8_t completeIndex = static_cast<uint8_t>(aComplete);
-  if (completeIndex < ArrayLength(PaymentCompleteValues::strings)) {
-    completeStatusString.AssignASCII(
-      PaymentCompleteValues::strings[completeIndex].value);
+  if (aTimedOut) {
+    completeStatusString.AssignLiteral("timeout");
+  } else {
+    uint8_t completeIndex = static_cast<uint8_t>(aComplete);
+    if (completeIndex < ArrayLength(PaymentCompleteValues::strings)) {
+      completeStatusString.AssignASCII(
+        PaymentCompleteValues::strings[completeIndex].value);
+    }
   }
 
   nsAutoString requestId;
diff --git a/dom/payments/PaymentRequestManager.h b/dom/payments/PaymentRequestManager.h
index 4225f8a4d8ad..47a20a9a3110 100644
--- a/dom/payments/PaymentRequestManager.h
+++ b/dom/payments/PaymentRequestManager.h
@@ -50,7 +50,8 @@ public:
   nsresult ShowPayment(PaymentRequest* aRequest);
   nsresult AbortPayment(PaymentRequest* aRequest, bool aDeferredShow);
   nsresult CompletePayment(PaymentRequest* aRequest,
-                           const PaymentComplete& aComplete);
+                           const PaymentComplete& aComplete,
+                           bool aTimedOut = false);
   nsresult UpdatePayment(JSContext* aCx,
                          PaymentRequest* aRequest,
                          const PaymentDetailsUpdate& aDetails,
diff --git a/dom/payments/PaymentResponse.cpp b/dom/payments/PaymentResponse.cpp
index 7fc264080302..c01d2b5b2882 100644
--- a/dom/payments/PaymentResponse.cpp
+++ b/dom/payments/PaymentResponse.cpp
@@ -4,6 +4,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#include "mozilla/StaticPrefs.h"
 #include "mozilla/dom/PaymentResponse.h"
 #include "mozilla/dom/BasicCardPaymentBinding.h"
 #include "BasicCardPayment.h"
@@ -22,6 +23,7 @@ NS_IMPL_CYCLE_COLLECTING_RELEASE(PaymentResponse)
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PaymentResponse)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
   NS_INTERFACE_MAP_ENTRY(nsISupports)
+  NS_INTERFACE_MAP_ENTRY(nsITimerCallback)
 NS_INTERFACE_MAP_END
 
 PaymentResponse::PaymentResponse(nsPIDOMWindowInner* aWindow,
@@ -49,6 +51,11 @@ PaymentResponse::PaymentResponse(nsPIDOMWindowInner* aWindow,
 
   // TODO: from https://github.com/w3c/browser-payment-api/issues/480
   // Add payerGivenName + payerFamilyName to PaymentAddress
+  NS_NewTimerWithCallback(getter_AddRefs(mTimer),
+                          this,
+                          StaticPrefs::dom_payments_response_timeout(),
+                          nsITimer::TYPE_ONE_SHOT,
+                          aWindow->EventTargetFor(TaskCategory::Other));
 }
 
 PaymentResponse::~PaymentResponse()
@@ -136,16 +143,13 @@ PaymentResponse::Complete(PaymentComplete result, ErrorResult& aRv)
     return nullptr;
   }
 
-  nsIGlobalObject* global = mOwner->AsGlobal();
-  ErrorResult errResult;
-  RefPtr<Promise> promise = Promise::Create(global, errResult);
-  if (errResult.Failed()) {
-    aRv.Throw(NS_ERROR_FAILURE);
-    return nullptr;
-  }
-
   mCompleteCalled = true;
 
+  if (mTimer) {
+    mTimer->Cancel();
+    mTimer = nullptr;
+  }
+
   RefPtr<PaymentRequestManager> manager = PaymentRequestManager::GetSingleton();
   if (NS_WARN_IF(!manager)) {
     aRv.Throw(NS_ERROR_FAILURE);
@@ -153,8 +157,16 @@ PaymentResponse::Complete(PaymentComplete result, ErrorResult& aRv)
   }
   nsresult rv = manager->CompletePayment(mRequest, result);
   if (NS_WARN_IF(NS_FAILED(rv))) {
-    promise->MaybeReject(NS_ERROR_FAILURE);
-    return promise.forget();
+    aRv.Throw(NS_ERROR_FAILURE);
+    return nullptr;
+  }
+
+  nsIGlobalObject* global = mOwner->AsGlobal();
+  ErrorResult errResult;
+  RefPtr<Promise> promise = Promise::Create(global, errResult);
+  if (errResult.Failed()) {
+    aRv.Throw(NS_ERROR_FAILURE);
+    return nullptr;
   }
 
   mPromise = promise;
@@ -164,10 +176,29 @@ PaymentResponse::Complete(PaymentComplete result, ErrorResult& aRv)
 void
 PaymentResponse::RespondComplete()
 {
-  MOZ_ASSERT(mPromise);
+  // mPromise may be null when timing out
+  if (mPromise) {
+    mPromise->MaybeResolve(JS::UndefinedHandleValue);
+    mPromise = nullptr;
+  }
+}
 
-  mPromise->MaybeResolve(JS::UndefinedHandleValue);
-  mPromise = nullptr;
+NS_IMETHODIMP
+PaymentResponse::Notify(nsITimer *timer)
+{
+  mTimer = nullptr;
+  if (mCompleteCalled) {
+    return NS_OK;
+  }
+
+  mCompleteCalled = true;
+
+  RefPtr<PaymentRequestManager> manager = PaymentRequestManager::GetSingleton();
+  if (NS_WARN_IF(!manager)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return manager->CompletePayment(mRequest, PaymentComplete::Unknown, true);
 }
 
 } // namespace dom
diff --git a/dom/payments/PaymentResponse.h b/dom/payments/PaymentResponse.h
index abee42963ad7..9bba1d80c8a5 100644
--- a/dom/payments/PaymentResponse.h
+++ b/dom/payments/PaymentResponse.h
@@ -10,6 +10,7 @@
 #include "mozilla/dom/PaymentResponseBinding.h" // PaymentComplete
 #include "nsPIDOMWindow.h"
 #include "nsWrapperCache.h"
+#include "nsITimer.h"
 
 namespace mozilla {
 namespace dom {
@@ -18,13 +19,15 @@ class PaymentAddress;
 class PaymentRequest;
 class Promise;
 
-class PaymentResponse final : public nsISupports,
+class PaymentResponse final : public nsITimerCallback,
                               public nsWrapperCache
 {
 public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(PaymentResponse)
 
+  NS_IMETHOD Notify(nsITimer* aTimer) override;
+
   PaymentResponse(nsPIDOMWindowInner* aWindow,
                   PaymentRequest* aRequest,
                   const nsAString& aRequestId,
@@ -83,6 +86,9 @@ private:
   RefPtr<PaymentAddress> mShippingAddress;
   // Promise for "PaymentResponse::Complete"
   RefPtr<Promise> mPromise;
+  // Timer for timing out if the page doesn't call
+  // complete()
+  nsCOMPtr<nsITimer> mTimer;
 };
 
 } // namespace dom
diff --git a/modules/libpref/init/StaticPrefList.h b/modules/libpref/init/StaticPrefList.h
index e0ab3ec72db7..98f93ac1d109 100644
--- a/modules/libpref/init/StaticPrefList.h
+++ b/modules/libpref/init/StaticPrefList.h
@@ -116,6 +116,14 @@ VARCACHE_PREF(
 // Note, this is not currently safe to use for normal browsing yet.
 PREF("dom.serviceWorkers.parent_intercept", bool, false)
 
+// Time in milliseconds for PaymentResponse to wait for
+// the Web page to call complete().
+VARCACHE_PREF(
+  "dom.payments.response.timeout",
+   dom_payments_response_timeout,
+  uint32_t, 5000
+)
+
 //---------------------------------------------------------------------------
 // Clear-Site-Data prefs
 //---------------------------------------------------------------------------