diff --git a/src/TextUI/Command.php b/src/TextUI/Command.php
index 65e44116976..967f2196095 100644
--- a/src/TextUI/Command.php
+++ b/src/TextUI/Command.php
@@ -779,6 +779,13 @@ protected function handleArguments(array $argv): void
             $this->arguments['testSuffixes'] = ['Test.php', '.phpt'];
         }
 
+        if (isset($this->options[1][0]) &&
+            \substr($this->options[1][0], -5, 5) !== '.phpt' &&
+            \substr($this->options[1][0], -4, 4) !== '.php'
+        ) {
+            print 'Warning: Calling PHPUnit with a class name is deprecated and will be removed in PHPUnit 9.' . \PHP_EOL;
+        }
+
         if (!isset($this->arguments['test'])) {
             if (isset($this->options[1][0])) {
                 $this->arguments['test'] = $this->options[1][0];