diff --git a/lib/eslint/flat-eslint.js b/lib/eslint/flat-eslint.js
index 479d5f14f2c..5866e445ec0 100644
--- a/lib/eslint/flat-eslint.js
+++ b/lib/eslint/flat-eslint.js
@@ -262,24 +262,16 @@ function findFlatConfigFile(cwd) {
 /**
  * Load the config array from the given filename.
  * @param {string} filePath The filename to load from.
- * @param {Object} options Options to help load the config file.
- * @param {string} options.basePath The base path for the config array.
- * @param {boolean} options.shouldIgnore Whether to honor ignore patterns.
- * @returns {Promise<FlatConfigArray>} The config array loaded from the config file.
+ * @returns {Promise<any>} The config loaded from the config file.
  */
-async function loadFlatConfigFile(filePath, { basePath, shouldIgnore }) {
+async function loadFlatConfigFile(filePath) {
     debug(`Loading config from ${filePath}`);
 
     const fileURL = pathToFileURL(filePath);
 
     debug(`Config file URL is ${fileURL}`);
 
-    const module = await import(fileURL);
-
-    return new FlatConfigArray(module.default, {
-        basePath,
-        shouldIgnore
-    });
+    return (await import(fileURL)).default;
 }
 
 /**
@@ -290,6 +282,7 @@ async function loadFlatConfigFile(filePath, { basePath, shouldIgnore }) {
  */
 async function calculateConfigArray(eslint, {
     cwd,
+    baseConfig,
     overrideConfig,
     configFile,
     ignore: shouldIgnore,
@@ -321,16 +314,18 @@ async function calculateConfigArray(eslint, {
         basePath = path.resolve(path.dirname(configFilePath));
     }
 
-    // load config array
-    let configs;
 
+    const configs = new FlatConfigArray(baseConfig || [], { basePath, shouldIgnore });
+
+    // load config file
     if (configFilePath) {
-        configs = await loadFlatConfigFile(configFilePath, {
-            basePath,
-            shouldIgnore
-        });
-    } else {
-        configs = new FlatConfigArray([], { basePath, shouldIgnore });
+        const fileConfig = await loadFlatConfigFile(configFilePath);
+
+        if (Array.isArray(fileConfig)) {
+            configs.push(...fileConfig);
+        } else {
+            configs.push(fileConfig);
+        }
     }
 
     // add in any configured defaults
diff --git a/tests/fixtures/eslint.config_with_rules.js b/tests/fixtures/eslint.config_with_rules.js
new file mode 100644
index 00000000000..6817217d965
--- /dev/null
+++ b/tests/fixtures/eslint.config_with_rules.js
@@ -0,0 +1,5 @@
+module.exports = [{
+    rules: {
+        quotes: ["error", "single"]
+    }
+}];
diff --git a/tests/lib/eslint/flat-eslint.js b/tests/lib/eslint/flat-eslint.js
index f4d28039503..60c0975a61c 100644
--- a/tests/lib/eslint/flat-eslint.js
+++ b/tests/lib/eslint/flat-eslint.js
@@ -4799,4 +4799,192 @@ describe("FlatESLint", () => {
         });
     });
 
+    describe("baseConfig", () => {
+        it("can be an object", async () => {
+            const eslint = new FlatESLint({
+                overrideConfigFile: true,
+                baseConfig: {
+                    rules: {
+                        semi: 2
+                    }
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText("foo");
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+        });
+
+        it("can be an array", async () => {
+            const eslint = new FlatESLint({
+                overrideConfigFile: true,
+                baseConfig: [
+                    {
+                        rules: {
+                            "no-var": 2
+                        }
+                    },
+                    {
+                        rules: {
+                            semi: 2
+                        }
+                    }
+                ]
+            });
+
+            const [{ messages }] = await eslint.lintText("var foo");
+
+            assert.strictEqual(messages.length, 2);
+            assert.strictEqual(messages[0].ruleId, "no-var");
+            assert.strictEqual(messages[1].ruleId, "semi");
+        });
+
+        it("should be inserted after default configs", async () => {
+            const eslint = new FlatESLint({
+                overrideConfigFile: true,
+                baseConfig: {
+                    languageOptions: {
+                        ecmaVersion: 5,
+                        sourceType: "script"
+                    }
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText("let x");
+
+            /*
+             * if baseConfig was inserted before default configs,
+             * `ecmaVersion: "latest"` from default configs would overwrite
+             * `ecmaVersion: 5` from baseConfig, so this wouldn't be a parsing error.
+             */
+
+            assert.strictEqual(messages.length, 1);
+            assert(messages[0].fatal, "Fatal error expected.");
+        });
+
+        it("should be inserted before configs from the config file", async () => {
+            const eslint = new FlatESLint({
+                cwd: getFixturePath(),
+                baseConfig: {
+                    rules: {
+                        strict: ["error", "global"]
+                    },
+                    languageOptions: {
+                        sourceType: "script"
+                    }
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText("foo");
+
+            /*
+             * if baseConfig was inserted after configs from the config file,
+             * `strict: 0` from eslint.config.js wouldn't overwrite `strict: ["error", "global"]`
+             * from baseConfig, so there would be an error message from the `strict` rule.
+             */
+
+            assert.strictEqual(messages.length, 0);
+        });
+
+        it("should be inserted before overrideConfig", async () => {
+            const eslint = new FlatESLint({
+                overrideConfigFile: true,
+                baseConfig: {
+                    rules: {
+                        semi: 2
+                    }
+                },
+                overrideConfig: {
+                    rules: {
+                        semi: 1
+                    }
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText("foo");
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+            assert.strictEqual(messages[0].severity, 1);
+        });
+
+        it("should be inserted before configs from the config file and overrideConfig", async () => {
+            const eslint = new FlatESLint({
+                overrideConfigFile: getFixturePath("eslint.config_with_rules.js"),
+                baseConfig: {
+                    rules: {
+                        quotes: ["error", "double"],
+                        semi: "error"
+                    }
+                },
+                overrideConfig: {
+                    rules: {
+                        quotes: "warn"
+                    }
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText('const foo = "bar"');
+
+            /*
+             * baseConfig: { quotes: ["error", "double"], semi: "error" }
+             * eslint.config_with_rules.js: { quotes: ["error", "single"] }
+             * overrideConfig: { quotes: "warn" }
+             *
+             * Merged config: { quotes: ["warn", "single"], semi: "error" }
+             */
+
+            assert.strictEqual(messages.length, 2);
+            assert.strictEqual(messages[0].ruleId, "quotes");
+            assert.strictEqual(messages[0].severity, 1);
+            assert.strictEqual(messages[1].ruleId, "semi");
+            assert.strictEqual(messages[1].severity, 2);
+        });
+
+        it("when it has 'files' they should be intepreted as relative to the config file", async () => {
+
+            /*
+             * `fixtures/plugins` directory does not have a config file.
+             * It's parent directory `fixtures` does have a config file, so
+             * the base path will be `fixtures`, cwd will be `fixtures/plugins`
+             */
+            const eslint = new FlatESLint({
+                cwd: getFixturePath("plugins"),
+                baseConfig: {
+                    files: ["plugins/a.js"],
+                    rules: {
+                        semi: 2
+                    }
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText("foo", { filePath: getFixturePath("plugins/a.js") });
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].ruleId, "semi");
+        });
+
+        it("when it has 'ignores' they should be intepreted as relative to the config file", async () => {
+
+            /*
+             * `fixtures/plugins` directory does not have a config file.
+             * It's parent directory `fixtures` does have a config file, so
+             * the base path will be `fixtures`, cwd will be `fixtures/plugins`
+             */
+            const eslint = new FlatESLint({
+                cwd: getFixturePath("plugins"),
+                baseConfig: {
+                    ignores: ["plugins/a.js"]
+                }
+            });
+
+            const [{ messages }] = await eslint.lintText("foo", { filePath: getFixturePath("plugins/a.js"), warnIgnored: true });
+
+            assert.strictEqual(messages.length, 1);
+            assert.strictEqual(messages[0].severity, 1);
+            assert.match(messages[0].message, /ignored/u);
+        });
+    });
+
 });