diff --git a/lib/rule-tester/flat-rule-tester.js b/lib/rule-tester/flat-rule-tester.js
index f297965ec33..5a93be1cce8 100644
--- a/lib/rule-tester/flat-rule-tester.js
+++ b/lib/rule-tester/flat-rule-tester.js
@@ -32,6 +32,7 @@ const { ConfigArraySymbol } = require("@humanwhocodes/config-array");
 
 /** @typedef {import("../shared/types").Parser} Parser */
 /** @typedef {import("../shared/types").LanguageOptions} LanguageOptions */
+/** @typedef {import("../shared/types").Rule} Rule */
 
 
 /**
@@ -446,7 +447,7 @@ class FlatRuleTester {
     /**
      * Adds a new rule test to execute.
      * @param {string} ruleName The name of the rule to run.
-     * @param {Function} rule The rule to test.
+     * @param {Function | Rule} rule The rule to test.
      * @param {{
      *   valid: (ValidTestCase | string)[],
      *   invalid: InvalidTestCase[]
diff --git a/lib/rule-tester/rule-tester.js b/lib/rule-tester/rule-tester.js
index a61f4028609..030a986867f 100644
--- a/lib/rule-tester/rule-tester.js
+++ b/lib/rule-tester/rule-tester.js
@@ -62,6 +62,7 @@ const { SourceCode } = require("../source-code");
 //------------------------------------------------------------------------------
 
 /** @typedef {import("../shared/types").Parser} Parser */
+/** @typedef {import("../shared/types").Rule} Rule */
 
 
 /**
@@ -508,17 +509,20 @@ class RuleTester {
     /**
      * Define a rule for one particular run of tests.
      * @param {string} name The name of the rule to define.
-     * @param {Function} rule The rule definition.
+     * @param {Function | Rule} rule The rule definition.
      * @returns {void}
      */
     defineRule(name, rule) {
+        if (typeof rule === "function") {
+            emitLegacyRuleAPIWarning(name);
+        }
         this.rules[name] = rule;
     }
 
     /**
      * Adds a new rule test to execute.
      * @param {string} ruleName The name of the rule to run.
-     * @param {Function} rule The rule to test.
+     * @param {Function | Rule} rule The rule to test.
      * @param {{
      *   valid: (ValidTestCase | string)[],
      *   invalid: InvalidTestCase[]