diff --git a/lib/internal/test_runner/harness.js b/lib/internal/test_runner/harness.js
index 285dd96d139653..26f5a6ef49142c 100644
--- a/lib/internal/test_runner/harness.js
+++ b/lib/internal/test_runner/harness.js
@@ -41,17 +41,21 @@ function createProcessEventHandler(eventName, rootTest) {
     // Check if this error is coming from a test. If it is, fail the test.
     const test = testResources.get(executionAsyncId());
 
-    if (!test) {
-      throw err;
-    }
-
-    if (test.finished) {
-      // If the test is already finished, report this as a top level
-      // diagnostic since this is a malformed test.
-      const msg = `Warning: Test "${test.name}" generated asynchronous ` +
-        'activity after the test ended. This activity created the error ' +
-        `"${err}" and would have caused the test to fail, but instead ` +
-        `triggered an ${eventName} event.`;
+    if (!test || test.finished) {
+      // If the test is already finished or the resource that created the error
+      // is not mapped to a Test, report this as a top level diagnostic.
+      let msg;
+
+      if (test) {
+        msg = `Warning: Test "${test.name}" generated asynchronous ` +
+          'activity after the test ended. This activity created the error ' +
+          `"${err}" and would have caused the test to fail, but instead ` +
+          `triggered an ${eventName} event.`;
+      } else {
+        msg = 'Warning: A resource generated asynchronous activity after ' +
+          `the test ended. This activity created the error "${err}" which ` +
+          `triggered an ${eventName} event, caught by the test runner.`;
+      }
 
       rootTest.diagnostic(msg);
       process.exitCode = kGenericUserError;
diff --git a/test/message/test_runner_output.js b/test/message/test_runner_output.js
index 593f069d8f4606..a53ded3dfde3ac 100644
--- a/test/message/test_runner_output.js
+++ b/test/message/test_runner_output.js
@@ -383,3 +383,9 @@ test('unfinished test with unhandledRejection', async () => {
     setTimeout(() => Promise.reject(new Error('bar')));
   });
 });
+
+// Verify that uncaught exceptions outside of any tests are handled after the
+// test harness has finished bootstrapping itself.
+setImmediate(() => {
+  throw new Error('uncaught from outside of a test');
+});
diff --git a/test/message/test_runner_output.out b/test/message/test_runner_output.out
index 2609833304e246..b6f254708010e9 100644
--- a/test/message/test_runner_output.out
+++ b/test/message/test_runner_output.out
@@ -637,6 +637,7 @@ not ok 65 - invalid subtest fail
 1..65
 # Warning: Test "unhandled rejection - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from unhandled rejection fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
 # Warning: Test "async unhandled rejection - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from async unhandled rejection fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
+# Warning: A resource generated asynchronous activity after the test ended. This activity created the error "Error: uncaught from outside of a test" which triggered an uncaughtException event, caught by the test runner.
 # Warning: Test "immediate throw - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: thrown from immediate throw fail" and would have caused the test to fail, but instead triggered an uncaughtException event.
 # Warning: Test "immediate reject - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from immediate reject fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
 # Warning: Test "callback called twice in different ticks" generated asynchronous activity after the test ended. This activity created the error "Error [ERR_TEST_FAILURE]: callback invoked multiple times" and would have caused the test to fail, but instead triggered an uncaughtException event.
diff --git a/test/message/test_runner_output_cli.out b/test/message/test_runner_output_cli.out
index 72957397c05454..3baeb534704b11 100644
--- a/test/message/test_runner_output_cli.out
+++ b/test/message/test_runner_output_cli.out
@@ -636,6 +636,7 @@ not ok 65 - invalid subtest fail
   ...
 # Warning: Test "unhandled rejection - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from unhandled rejection fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
 # Warning: Test "async unhandled rejection - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from async unhandled rejection fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
+# Warning: A resource generated asynchronous activity after the test ended. This activity created the error "Error: uncaught from outside of a test" which triggered an uncaughtException event, caught by the test runner.
 # Warning: Test "immediate throw - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: thrown from immediate throw fail" and would have caused the test to fail, but instead triggered an uncaughtException event.
 # Warning: Test "immediate reject - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from immediate reject fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
 # Warning: Test "callback called twice in different ticks" generated asynchronous activity after the test ended. This activity created the error "Error [ERR_TEST_FAILURE]: callback invoked multiple times" and would have caused the test to fail, but instead triggered an uncaughtException event.
diff --git a/test/message/test_runner_output_spec_reporter.out b/test/message/test_runner_output_spec_reporter.out
index 591dc9a76d64d0..3ff9aefe7a42ce 100644
--- a/test/message/test_runner_output_spec_reporter.out
+++ b/test/message/test_runner_output_spec_reporter.out
@@ -270,6 +270,7 @@
 
  Warning: Test "unhandled rejection - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from unhandled rejection fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
  Warning: Test "async unhandled rejection - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from async unhandled rejection fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
+ Warning: A resource generated asynchronous activity after the test ended. This activity created the error "Error: uncaught from outside of a test" which triggered an uncaughtException event, caught by the test runner.
  Warning: Test "immediate throw - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: thrown from immediate throw fail" and would have caused the test to fail, but instead triggered an uncaughtException event.
  Warning: Test "immediate reject - passes but warns" generated asynchronous activity after the test ended. This activity created the error "Error: rejected from immediate reject fail" and would have caused the test to fail, but instead triggered an unhandledRejection event.
  Warning: Test "callback called twice in different ticks" generated asynchronous activity after the test ended. This activity created the error "Error [ERR_TEST_FAILURE]: callback invoked multiple times" and would have caused the test to fail, but instead triggered an uncaughtException event.