diff --git a/lib/cli/run-helpers.js b/lib/cli/run-helpers.js
index dfac8b4479..d0f9e74635 100644
--- a/lib/cli/run-helpers.js
+++ b/lib/cli/run-helpers.js
@@ -195,10 +195,10 @@ exports.runMocha = async (mocha, options) => {
  * it actually exists. This must be run _after_ requires are processed (see
  * {@link handleRequires}), as it'll prevent interfaces from loading otherwise.
  * @param {Object} opts - Options object
- * @param {"reporter"|"interface"} pluginType - Type of plugin.
- * @param {Object} [map] - An object perhaps having key `key`. Used as a cache
- * of sorts; `Mocha.reporters` is one, where each key corresponds to a reporter
- * name
+ * @param {"reporter"|"ui"} pluginType - Type of plugin.
+ * @param {Object} [map] - Used as a cache of sorts;
+ * `Mocha.reporters` where each key corresponds to a reporter name,
+ * `Mocha.interfaces` where each key corresponds to an interface name.
  * @private
  */
 exports.validateLegacyPlugin = (opts, pluginType, map = {}) => {
@@ -226,12 +226,12 @@ exports.validateLegacyPlugin = (opts, pluginType, map = {}) => {
   // if this exists, then it's already loaded, so nothing more to do.
   if (!map[pluginId]) {
     try {
-      opts[pluginType] = require(pluginId);
+      map[pluginId] = require(pluginId);
     } catch (err) {
       if (err.code === 'MODULE_NOT_FOUND') {
         // Try to load reporters from a path (absolute or relative)
         try {
-          opts[pluginType] = require(path.resolve(pluginId));
+          map[pluginId] = require(path.resolve(pluginId));
         } catch (err) {
           throw createUnknownError(err);
         }
diff --git a/lib/errors.js b/lib/errors.js
index 681198ae27..0f0940b481 100644
--- a/lib/errors.js
+++ b/lib/errors.js
@@ -328,7 +328,7 @@ function createFatalError(message, value) {
 /**
  * Dynamically creates a plugin-type-specific error based on plugin type
  * @param {string} message - Error message
- * @param {"reporter"|"interface"} pluginType - Plugin type. Future: expand as needed
+ * @param {"reporter"|"ui"} pluginType - Plugin type. Future: expand as needed
  * @param {string} [pluginId] - Name/path of plugin, if any
  * @throws When `pluginType` is not known
  * @public
@@ -339,7 +339,7 @@ function createInvalidLegacyPluginError(message, pluginType, pluginId) {
   switch (pluginType) {
     case 'reporter':
       return createInvalidReporterError(message, pluginId);
-    case 'interface':
+    case 'ui':
       return createInvalidInterfaceError(message, pluginId);
     default:
       throw new Error('unknown pluginType "' + pluginType + '"');
diff --git a/test/node-unit/cli/run-helpers.spec.js b/test/node-unit/cli/run-helpers.spec.js
index e12b97b875..9c3f60ee8a 100644
--- a/test/node-unit/cli/run-helpers.spec.js
+++ b/test/node-unit/cli/run-helpers.spec.js
@@ -1,6 +1,7 @@
 'use strict';
 
 const {validateLegacyPlugin, list} = require('../../../lib/cli/run-helpers');
+const Mocha = require('../../../lib/mocha');
 
 describe('helpers', function() {
   describe('validateLegacyPlugin()', function() {
@@ -25,24 +26,19 @@ describe('helpers', function() {
       });
     });
 
-    describe('when used with an "interfaces" key', function() {
+    describe('when used with an "ui" key', function() {
       it('should disallow an array of names', function() {
-        expect(
-          () => validateLegacyPlugin({interface: ['bar']}, 'interface'),
-          'to throw',
-          {
-            code: 'ERR_MOCHA_INVALID_INTERFACE',
-            message: /can only be specified once/i
-          }
-        );
+        expect(() => validateLegacyPlugin({ui: ['bar']}, 'ui'), 'to throw', {
+          code: 'ERR_MOCHA_INVALID_INTERFACE',
+          message: /can only be specified once/i
+        });
       });
 
       it('should fail to recognize an unknown interface', function() {
-        expect(
-          () => validateLegacyPlugin({interface: 'bar'}, 'interface'),
-          'to throw',
-          {code: 'ERR_MOCHA_INVALID_INTERFACE', message: /cannot find module/i}
-        );
+        expect(() => validateLegacyPlugin({ui: 'bar'}, 'ui'), 'to throw', {
+          code: 'ERR_MOCHA_INVALID_INTERFACE',
+          message: /cannot find module/i
+        });
       });
     });
 
@@ -56,6 +52,17 @@ describe('helpers', function() {
       });
     });
 
+    describe('when used with a third-party interface', function() {
+      it('should add the interface to "Mocha.interfaces"', function() {
+        // let's suppose that `glob` is an interface
+        const opts = {ui: 'glob'};
+        validateLegacyPlugin(opts, 'ui', Mocha.interfaces);
+        expect(opts.ui, 'to equal', 'glob');
+        expect(Mocha.interfaces, 'to satisfy', {glob: require('glob')});
+        delete Mocha.interfaces.glob;
+      });
+    });
+
     describe('when a plugin throws an exception upon load', function() {
       it('should fail and report the original error', function() {
         expect(