diff --git a/README.md b/README.md
index 6709896..bbf5551 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,10 @@ new PHPFileBuilder(app_path('Models/User.php'))
 
 ### Features
 
+#### setNamespace
+
+Set or replace the namespace declaration in a PHP file. If the file has no namespace, it will be added. If the namespace is already the same, no changes are made.
+
 #### setProperty
 
 Add new class property with the passed value and passed access level in case property does not exist in the class. Otherwise
diff --git a/src/Builders/PHPFileBuilder.php b/src/Builders/PHPFileBuilder.php
index 0f9128e..5644de7 100644
--- a/src/Builders/PHPFileBuilder.php
+++ b/src/Builders/PHPFileBuilder.php
@@ -16,6 +16,7 @@
 use RonasIT\Larabuilder\Visitors\PropertyVisitors\AddArrayPropertyItem;
 use RonasIT\Larabuilder\Visitors\PropertyVisitors\RemoveArrayPropertyItem;
 use RonasIT\Larabuilder\Visitors\PropertyVisitors\SetProperty;
+use RonasIT\Larabuilder\Visitors\SetNamespace;
 
 class PHPFileBuilder
 {
@@ -41,6 +42,13 @@ public function __construct(
         $this->traverser = new NodeTraverser();
     }
 
+    public function setNamespace(string $namespace): self
+    {
+        $this->traverser->addVisitor(new SetNamespace($namespace));
+
+        return $this;
+    }
+
     public function setProperty(string $name, mixed $value, AccessModifierEnum $accessModifier = AccessModifierEnum::Public): self
     {
         $this->traverser->addVisitor(new SetProperty($name, $value, $accessModifier));
diff --git a/src/Visitors/SetNamespace.php b/src/Visitors/SetNamespace.php
new file mode 100644
index 0000000..b3ddf90
--- /dev/null
+++ b/src/Visitors/SetNamespace.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace RonasIT\Larabuilder\Visitors;
+
+use PhpParser\Node\Name;
+use PhpParser\Node\Stmt\Declare_;
+use PhpParser\Node\Stmt\Namespace_;
+use PhpParser\NodeVisitorAbstract;
+
+class SetNamespace extends NodeVisitorAbstract
+{
+    public function __construct(
+        protected string $namespace,
+    ) {
+    }
+
+    public function afterTraverse(array $nodes): ?array
+    {
+        $declares = [];
+
+        foreach ($nodes as $key => $node) {
+            if ($node instanceof Namespace_) {
+                return $this->updateNamespace($node);
+            }
+
+            if ($node instanceof Declare_) {
+                $declares[] = $node;
+
+                unset($nodes[$key]);
+            }
+        }
+
+        return [...$declares, new Namespace_(new Name($this->namespace), array_values($nodes))];
+    }
+
+    protected function updateNamespace(Namespace_ $node): ?array
+    {
+        if ($node->name->toString() !== $this->namespace) {
+            $node->name = new Name($this->namespace);
+        }
+
+        return null;
+    }
+}
diff --git a/tests/PHPFileBuilderTest.php b/tests/PHPFileBuilderTest.php
index d6148d1..b834d08 100644
--- a/tests/PHPFileBuilderTest.php
+++ b/tests/PHPFileBuilderTest.php
@@ -17,6 +17,48 @@ class PHPFileBuilderTest extends TestCase
 {
     use PHPFileBuilderTestMockTrait;
 
+    public function testSetNamespaceOnFileWithoutNamespace(): void
+    {
+        $file = $this->generateOriginalStructurePath('class_empty.php');
+
+        $this->mockNativeFunction(
+            'RonasIT\Larabuilder\Builders',
+            $this->callFilePutContent($file, 'namespace_set.php'),
+        );
+
+        new PHPFileBuilder($file)
+            ->setNamespace('App\\Models')
+            ->save();
+    }
+
+    public function testSetNamespaceReplacesExistingNamespace(): void
+    {
+        $file = $this->generateOriginalStructurePath('enum.php');
+
+        $this->mockNativeFunction(
+            'RonasIT\Larabuilder\Builders',
+            $this->callFilePutContent($file, 'namespace_update.php'),
+        );
+
+        new PHPFileBuilder($file)
+            ->setNamespace('App\\Models')
+            ->save();
+    }
+
+    public function testSetNamespaceDoesNothingWhenSameNamespace(): void
+    {
+        $file = $this->generateOriginalStructurePath('class.php');
+
+        $this->mockNativeFunction(
+            'RonasIT\Larabuilder\Builders',
+            $this->callFilePutContent($file, 'class_unchanged.php'),
+        );
+
+        new PHPFileBuilder($file)
+            ->setNamespace('RonasIT\Larabuilder\Tests\Support')
+            ->save();
+    }
+
     public function testSetProperty(): void
     {
         $file = $this->generateOriginalStructurePath('class_with_properties.php');
diff --git a/tests/Support/OriginStructures/class_empty.php b/tests/Support/OriginStructures/class_empty.php
new file mode 100644
index 0000000..0cf3f47
--- /dev/null
+++ b/tests/Support/OriginStructures/class_empty.php
@@ -0,0 +1,7 @@
+<?php
+
+declare(strict_types=1);
+
+class SomeClass
+{
+}
diff --git a/tests/fixtures/PHPFileBuilderTest/namespace_set.php b/tests/fixtures/PHPFileBuilderTest/namespace_set.php
new file mode 100644
index 0000000..263fe27
--- /dev/null
+++ b/tests/fixtures/PHPFileBuilderTest/namespace_set.php
@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+class SomeClass
+{
+}
diff --git a/tests/fixtures/PHPFileBuilderTest/namespace_update.php b/tests/fixtures/PHPFileBuilderTest/namespace_update.php
new file mode 100644
index 0000000..ca7ba21
--- /dev/null
+++ b/tests/fixtures/PHPFileBuilderTest/namespace_update.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Models;
+
+enum SomeEnum
+{
+    case First;
+    case Second;
+
+    public static function toArray(): array
+    {
+        return self::cases();
+    }
+}