diff --git a/src/Symfony/Component/HttpFoundation/Response.php b/src/Symfony/Component/HttpFoundation/Response.php
index 6766f2c77099e..455b026dffb05 100644
--- a/src/Symfony/Component/HttpFoundation/Response.php
+++ b/src/Symfony/Component/HttpFoundation/Response.php
@@ -261,7 +261,7 @@ public function prepare(Request $request): static
             }
 
             // Fix Content-Type
-            $charset = $this->charset ?: 'UTF-8';
+            $charset = $this->charset ?: 'utf-8';
             if (!$headers->has('Content-Type')) {
                 $headers->set('Content-Type', 'text/html; charset='.$charset);
             } elseif (0 === stripos($headers->get('Content-Type') ?? '', 'text/') && false === stripos($headers->get('Content-Type') ?? '', 'charset')) {
diff --git a/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php b/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php
index 2c761a4f8ad17..26ce83df1c48d 100644
--- a/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php
+++ b/src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php
@@ -63,7 +63,7 @@ public function testSend()
     public function testGetCharset()
     {
         $response = new Response();
-        $charsetOrigin = 'UTF-8';
+        $charsetOrigin = 'utf-8';
         $response->setCharset($charsetOrigin);
         $charset = $response->getCharset();
         $this->assertEquals($charsetOrigin, $charset);
@@ -534,7 +534,7 @@ public function testDefaultContentType()
         $response = new Response('foo');
         $response->prepare(new Request());
 
-        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('Content-Type'));
+        $this->assertSame('text/html; charset=utf-8', $response->headers->get('Content-Type'));
     }
 
     public function testContentTypeCharset()
@@ -545,7 +545,7 @@ public function testContentTypeCharset()
         // force fixContentType() to be called
         $response->prepare(new Request());
 
-        $this->assertEquals('text/css; charset=UTF-8', $response->headers->get('Content-Type'));
+        $this->assertEquals('text/css; charset=utf-8', $response->headers->get('Content-Type'));
     }
 
     public function testContentTypeIsNull()
@@ -565,7 +565,7 @@ public function testPrepareDoesNothingIfContentTypeIsSet()
 
         $response->prepare(new Request());
 
-        $this->assertEquals('text/plain; charset=UTF-8', $response->headers->get('content-type'));
+        $this->assertEquals('text/plain; charset=utf-8', $response->headers->get('content-type'));
     }
 
     public function testPrepareDoesNothingIfRequestFormatIsNotDefined()
@@ -574,7 +574,7 @@ public function testPrepareDoesNothingIfRequestFormatIsNotDefined()
 
         $response->prepare(new Request());
 
-        $this->assertEquals('text/html; charset=UTF-8', $response->headers->get('content-type'));
+        $this->assertEquals('text/html; charset=utf-8', $response->headers->get('content-type'));
     }
 
     /**
@@ -588,7 +588,7 @@ public function testPrepareDoesNotSetContentTypeBasedOnRequestAcceptHeader()
         $request->headers->set('Accept', 'application/json');
         $response->prepare($request);
 
-        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('content-type'));
+        $this->assertSame('text/html; charset=utf-8', $response->headers->get('content-type'));
     }
 
     public function testPrepareSetContentType()
@@ -1021,7 +1021,7 @@ public function testSettersAreChainable()
 
         $setters = [
             'setProtocolVersion' => '1.0',
-            'setCharset' => 'UTF-8',
+            'setCharset' => 'utf-8',
             'setPublic' => null,
             'setPrivate' => null,
             'setDate' => $this->createDateTimeNow(),