diff --git a/js/src/builtin/Promise.cpp b/js/src/builtin/Promise.cpp
index 8cdf41f400f1..30b342279ac0 100644
--- a/js/src/builtin/Promise.cpp
+++ b/js/src/builtin/Promise.cpp
@@ -3909,6 +3909,19 @@ bool js::IsPromiseForAsync(JSObject* promise) {
 MOZ_MUST_USE bool js::AsyncFunctionThrown(JSContext* cx,
                                           Handle<PromiseObject*> resultPromise,
                                           HandleValue reason) {
+  if (resultPromise->state() != JS::PromiseState::Pending) {
+    // OOM after resolving promise.
+    // Report a warning and ignore the result.
+    if (!JS_ReportErrorFlagsAndNumberASCII(
+            cx, JSREPORT_WARNING, GetErrorMessage, nullptr,
+            JSMSG_UNHANDLABLE_PROMISE_REJECTION_WARNING)) {
+      if (cx->isExceptionPending()) {
+        cx->clearPendingException();
+      }
+    }
+    return true;
+  }
+
   return RejectPromiseInternal(cx, resultPromise, reason);
 }
 
diff --git a/js/src/jit-test/tests/async/debugger-reject-after-fulfill.js b/js/src/jit-test/tests/async/debugger-reject-after-fulfill.js
new file mode 100644
index 000000000000..4b3637d2350f
--- /dev/null
+++ b/js/src/jit-test/tests/async/debugger-reject-after-fulfill.js
@@ -0,0 +1,227 @@
+// Throwing error after resolving async function's promise should not
+// overwrite the promise's state or value/reason.
+// This situation can happen either with debugger interaction or OOM.
+
+// This testcase relies on the fact that there's a breakpoint after resolving
+// the async function's promise, before leaving the async function's frame.
+// This function searches for the last breakpoint before leaving the frame.
+//
+//  - `declCode` should declare an async function `f`, and the function should
+//    set global variable `returning` to `true` just before return
+//  - `callCode` should call the function `f` and make sure the function's
+//    execution reaches the last breakpoint
+function searchLastBreakpointBeforeReturn(declCode, callCode) {
+  const g = newGlobal({ newCompartment: true });
+  const dbg = new Debugger(g);
+  g.eval(declCode);
+
+  let hit = false;
+  let offset = 0;
+  dbg.onEnterFrame = function(frame) {
+    if (frame.callee && frame.callee.name == "f") {
+      frame.onStep = () => {
+        if (!g.returning) {
+          return undefined;
+        }
+
+        offset = frame.offset;
+        return undefined;
+      };
+    }
+  };
+
+  g.eval(callCode);
+
+  drainJobQueue();
+
+  assertEq(offset != 0, true);
+
+  return offset;
+}
+
+function testWithoutAwait() {
+  const declCode = `
+  var returning = false;
+  async function f() {
+    return (returning = true, "expected");
+  };
+  `;
+
+  const callCode = `
+  var p = f();
+  `;
+
+  const offset = searchLastBreakpointBeforeReturn(declCode, callCode);
+
+  const g = newGlobal({ newCompartment: true });
+  const dbg = new Debugger(g);
+  g.eval(declCode);
+
+  let onPromiseSettledCount = 0;
+  dbg.onPromiseSettled = function(promise) {
+    onPromiseSettledCount++;
+    // No promise should be rejected.
+    assertEq(promise.promiseState, "fulfilled");
+
+    // Async function's promise should have expected value.
+    if (onPromiseSettledCount == 1) {
+      assertEq(promise.promiseValue, "expected");
+    }
+  };
+
+  let hitBreakpoint = false;
+  dbg.onEnterFrame = function(frame) {
+    if (frame.callee && frame.callee.name == "f") {
+      frame.script.setBreakpoint(offset, {
+        hit() {
+          hitBreakpoint = true;
+          return { throw: "unexpected" };
+        }
+      });
+    }
+  };
+
+  enableLastWarning();
+
+  g.eval(`
+  var fulfilledValue;
+  var rejected = false;
+  `);
+
+  g.eval(callCode);
+
+  // The execution reaches to the last breakpoint without running job queue.
+  assertEq(hitBreakpoint, true);
+
+  const warn = getLastWarning();
+  assertEq(warn.message,
+           "unhandlable error after resolving async function's promise");
+  clearLastWarning();
+
+  // Add reaction handler after resolution.
+  // This handler's job will be enqueued immediately.
+  g.eval(`
+  p.then(x => {
+    fulfilledValue = x;
+  }, e => {
+    rejected = true;
+  });
+  `);
+
+  // Run the above handler.
+  drainJobQueue();
+
+  assertEq(g.fulfilledValue, "expected");
+  assertEq(onPromiseSettledCount >= 1, true);
+}
+
+function testWithAwait() {
+  const declCode = `
+  var resolve;
+  var p = new Promise(r => { resolve = r });
+  var returning = false;
+  async function f() {
+    await p;
+    return (returning = true, "expected");
+  };
+  `;
+
+  const callCode = `
+  var p = f();
+  `;
+
+  const resolveCode = `
+  resolve("resolve");
+  `;
+
+  const offset = searchLastBreakpointBeforeReturn(declCode,
+                                                  callCode + resolveCode);
+
+  const g = newGlobal({newCompartment: true});
+  const dbg = new Debugger(g);
+  g.eval(declCode);
+
+  let onPromiseSettledCount = 0;
+  dbg.onPromiseSettled = function(promise) {
+    onPromiseSettledCount++;
+
+    // No promise should be rejected.
+    assertEq(promise.promiseState, "fulfilled");
+
+    // Async function's promise should have expected value.
+    if (onPromiseSettledCount == 3) {
+      assertEq(promise.promiseValue, "expected");
+    }
+  };
+
+  let hitBreakpoint = false;
+  dbg.onEnterFrame = function(frame) {
+    if (frame.callee && frame.callee.name == "f") {
+      frame.script.setBreakpoint(offset, {
+        hit() {
+          hitBreakpoint = true;
+          return { throw: "unexpected" };
+        }
+      });
+    }
+  };
+
+  enableLastWarning();
+
+  g.eval(`
+  var fulfilledValue1;
+  var fulfilledValue2;
+  var rejected = false;
+  `);
+
+  g.eval(callCode);
+
+  assertEq(getLastWarning(), null);
+
+  // Add reaction handler before resolution.
+  // This handler's job will be enqueued when `p` is resolved.
+  g.eval(`
+  p.then(x => {
+    fulfilledValue1 = x;
+  }, e => {
+    rejected = true;
+  });
+  `);
+
+  g.eval(resolveCode);
+
+  // Run the remaining part of async function, and the above handler.
+  drainJobQueue();
+
+  // The execution reaches to the last breakpoint after running job queue for
+  // resolving `p`.
+  assertEq(hitBreakpoint, true);
+
+  const warn = getLastWarning();
+  assertEq(warn.message,
+           "unhandlable error after resolving async function's promise");
+  clearLastWarning();
+
+  assertEq(g.fulfilledValue1, "expected");
+  assertEq(g.rejected, false);
+
+  // Add reaction handler after resolution.
+  // This handler's job will be enqueued immediately.
+  g.eval(`
+  p.then(x => {
+    fulfilledValue2 = x;
+  }, e => {
+    rejected = true;
+  });
+  `);
+
+  // Run the above handler.
+  drainJobQueue();
+
+  assertEq(g.fulfilledValue2, "expected");
+  assertEq(g.rejected, false);
+  assertEq(onPromiseSettledCount >= 3, true);
+}
+
+testWithoutAwait();
+testWithAwait();
diff --git a/js/src/js.msg b/js/src/js.msg
index 1417d84f900b..c4b7a726231f 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -644,6 +644,9 @@ MSG_DEF(JSMSG_PROMISE_ERROR_IN_WRAPPED_REJECTION_REASON,0, JSEXN_INTERNALERR, "P
 MSG_DEF(JSMSG_RETURN_NOT_CALLABLE,     0, JSEXN_TYPEERR, "property 'return' of iterator is not callable")
 MSG_DEF(JSMSG_ITERATOR_NO_THROW,       0, JSEXN_TYPEERR, "iterator does not have a 'throw' method")
 
+// Async Function
+MSG_DEF(JSMSG_UNHANDLABLE_PROMISE_REJECTION_WARNING, 0, JSEXN_WARN, "unhandlable error after resolving async function's promise")
+
 // Async Iteration
 MSG_DEF(JSMSG_FOR_AWAIT_NOT_OF,        0, JSEXN_SYNTAXERR, "'for await' loop should be used with 'of'")
 MSG_DEF(JSMSG_NOT_AN_ASYNC_GENERATOR,  0, JSEXN_TYPEERR, "Not an async generator")