diff --git a/lib/test.js b/lib/test.js
index 303e0d2e7..7d2d5139b 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -9,6 +9,16 @@ const nowAndTimers = require('./now-and-timers');
 const parseTestArgs = require('./parse-test-args');
 const concordanceOptions = require('./concordance-options').default;
 
+function isExternalAssertError(error) {
+	// Match errors thrown by <https://www.npmjs.com/package/expect>.
+	if (error.matcherResult) {
+		return error.matcherResult.actual && error.matcherResult.expected;
+	}
+
+	// Match errors thrown by <https://www.npmjs.com/package/chai> and <https://nodejs.org/api/assert.html>.
+	return error.actual && error.expected;
+}
+
 function formatErrorValue(label, error) {
 	const formatted = concordance.format(error, concordanceOptions);
 	return {label, formatted};
@@ -565,11 +575,19 @@ class Test {
 		const result = this.callFn();
 		if (!result.ok) {
 			if (!this.detectImproperThrows(result.error)) {
-				this.saveFirstError(new assert.AssertionError({
-					message: 'Error thrown in test',
-					savedError: result.error instanceof Error && result.error,
-					values: [formatErrorValue('Error thrown in test:', result.error)]
-				}));
+				if (isExternalAssertError(result.error)) {
+					this.saveFirstError(new assert.AssertionError({
+						message: 'Assertion failed',
+						savedError: result.error instanceof Error && result.error,
+						values: [{label: 'Assertion failed: ', formatted: result.error.message}]
+					}));
+				} else {
+					this.saveFirstError(new assert.AssertionError({
+						message: 'Error thrown in test',
+						savedError: result.error instanceof Error && result.error,
+						values: [formatErrorValue('Error thrown in test:', result.error)]
+					}));
+				}
 			}
 
 			return this.finishPromised();
@@ -638,11 +656,19 @@ class Test {
 				promise
 					.catch(error => {
 						if (!this.detectImproperThrows(error)) {
-							this.saveFirstError(new assert.AssertionError({
-								message: 'Rejected promise returned by test',
-								savedError: error instanceof Error && error,
-								values: [formatErrorValue('Rejected promise returned by test. Reason:', error)]
-							}));
+							if (isExternalAssertError(error)) {
+								this.saveFirstError(new assert.AssertionError({
+									message: 'Assertion failed',
+									savedError: error instanceof Error && error,
+									values: [{label: 'Assertion failed: ', formatted: error.message}]
+								}));
+							} else {
+								this.saveFirstError(new assert.AssertionError({
+									message: 'Rejected promise returned by test',
+									savedError: error instanceof Error && error,
+									values: [formatErrorValue('Rejected promise returned by test. Reason:', error)]
+								}));
+							}
 						}
 					})
 					.then(() => resolve(this.finishPromised())); // eslint-disable-line promise/prefer-await-to-then