diff --git a/lib/cli/run.js b/lib/cli/run.js
index 94c7c991fb..cf56f3ac07 100644
--- a/lib/cli/run.js
+++ b/lib/cli/run.js
@@ -279,6 +279,13 @@ exports.builder = yargs =>
         );
       }
 
+      if (argv.opts) {
+        throw createUnsupportedError(
+          `--opts: configuring Mocha via 'mocha.opts' is DEPRECATED and no longer supported.
+          Please use a configuration file instead.`
+        );
+      }
+
       // load requires first, because it can impact "plugin" validation
       handleRequires(argv.require);
       validatePlugin(argv, 'reporter', Mocha.reporters);