diff --git a/src/Symfony/Bundle/Test/ApiTestCase.php b/src/Symfony/Bundle/Test/ApiTestCase.php
index 891b5f09ab9..90bd2a7db96 100644
--- a/src/Symfony/Bundle/Test/ApiTestCase.php
+++ b/src/Symfony/Bundle/Test/ApiTestCase.php
@@ -105,7 +105,7 @@ protected static function createClient(array $kernelOptions = [], array $default
             throw new \LogicException('You cannot create the client used in functional tests if the "framework.test" config is not set to true.');
         }
 
-        $client->setDefaultOptions($defaultOptions);
+        $client->setDefaultOptions(self::withDefaultContentType($defaultOptions));
 
         self::getHttpClient($client);
         self::getClient($client->getKernelBrowser());
@@ -113,6 +113,54 @@ protected static function createClient(array $kernelOptions = [], array $default
         return $client;
     }
 
+    /**
+     * Symfony's HttpClient adds "Content-Type: application/json" automatically when the "json" option is used.
+     * On a default API Platform project, "json" is not part of the configured formats, so such requests fail
+     * with 415 Unsupported Media Type. To keep tests working out of the box in that scenario, default the
+     * Content-Type to the first configured API Platform format.
+     *
+     * The default is only applied when "application/json" is NOT one of the configured mime types: when it is
+     * (e.g. a project that explicitly enables the "json" format, or any GraphQL endpoint that accepts
+     * "application/json" regardless of the API Platform formats), Symfony's implicit "application/json"
+     * header already produces a valid request, and overriding it would break per-request "json" usage.
+     */
+    private static function withDefaultContentType(array $defaultOptions): array
+    {
+        $headers = $defaultOptions['headers'] ?? [];
+        foreach (array_keys($headers) as $name) {
+            if (\is_string($name) && 0 === strcasecmp($name, 'content-type')) {
+                return $defaultOptions;
+            }
+        }
+
+        $container = self::getContainer();
+        if (!$container->hasParameter('api_platform.formats')) {
+            return $defaultOptions;
+        }
+
+        $formats = $container->getParameter('api_platform.formats');
+        if (!\is_array($formats) || !$formats) {
+            return $defaultOptions;
+        }
+
+        foreach ($formats as $mimeTypes) {
+            if (\is_array($mimeTypes) && \in_array('application/json', $mimeTypes, true)) {
+                return $defaultOptions;
+            }
+        }
+
+        $firstFormat = reset($formats);
+        $mimeType = \is_array($firstFormat) ? ($firstFormat[0] ?? null) : null;
+        if (!\is_string($mimeType) || '' === $mimeType) {
+            return $defaultOptions;
+        }
+
+        $headers['content-type'] = $mimeType;
+        $defaultOptions['headers'] = $headers;
+
+        return $defaultOptions;
+    }
+
     /**
      * Finds the IRI of a resource item matching the resource class and the specified criteria.
      */
diff --git a/tests/Symfony/Bundle/Test/ApiTestCaseTest.php b/tests/Symfony/Bundle/Test/ApiTestCaseTest.php
index 422d1e558c6..3756bb92998 100644
--- a/tests/Symfony/Bundle/Test/ApiTestCaseTest.php
+++ b/tests/Symfony/Bundle/Test/ApiTestCaseTest.php
@@ -403,6 +403,28 @@ public function testMissingMethod(): void
         $this->assertResponseStatusCodeSame(404);
     }
 
+    public function testJsonOptionContentTypePreservedWhenApplicationJsonConfigured(): void
+    {
+        // The test fixtures configure "application/json" among the API Platform formats, so Symfony's
+        // implicit "Content-Type: application/json" header (set by the "json" option) must not be overridden.
+        // Overriding it would break callers that legitimately rely on application/json (e.g. /graphql).
+        $client = self::createClient();
+        $client->request('POST', '/something/that/does/not/exist/ever', ['json' => ['name' => 'Kevin']]);
+
+        $this->assertSame('application/json', $client->getKernelBrowser()->getRequest()->headers->get('Content-Type'));
+    }
+
+    public function testExplicitContentTypeIsPreserved(): void
+    {
+        $client = self::createClient();
+        $client->request('POST', '/something/that/does/not/exist/ever', [
+            'headers' => ['content-type' => 'application/json'],
+            'body' => '{"name":"Kevin"}',
+        ]);
+
+        $this->assertSame('application/json', $client->getKernelBrowser()->getRequest()->headers->get('Content-Type'));
+    }
+
     public function testDoNotRebootKernelOnCreateClient(): void
     {
         self::$alwaysBootKernel = false;