diff --git a/app/Config/Autoload.php b/app/Config/Autoload.php
index 76cd9263556e..784121fdd04d 100644
--- a/app/Config/Autoload.php
+++ b/app/Config/Autoload.php
@@ -40,7 +40,7 @@ class Autoload extends AutoloadConfig
      * @var array<string, list<string>|string>
      */
     public $psr4 = [
-        APP_NAMESPACE => APPPATH,
+        APP_NAMESPACE => [APPPATH, TESTPATH . 'app'],
     ];
 
     /**
diff --git a/system/Autoloader/Autoloader.php b/system/Autoloader/Autoloader.php
index 4c6fed350fe2..c6ddd84f2efc 100644
--- a/system/Autoloader/Autoloader.php
+++ b/system/Autoloader/Autoloader.php
@@ -63,14 +63,14 @@ class Autoloader
     /**
      * Stores namespaces as key, and path as values.
      *
-     * @var array<string, array<string>>
+     * @var array<string, list<string>>
      */
     protected $prefixes = [];
 
     /**
      * Stores class name as key, and path as values.
      *
-     * @var array<string, string>
+     * @var array<class-string, string>
      */
     protected $classmap = [];
 
@@ -215,7 +215,8 @@ public function addNamespace($namespace, ?string $path = null)
      *
      * If a prefix param is set, returns only paths to the given prefix.
      *
-     * @return array
+     * @return array<string, list<string>>|list<string>
+     * @phpstan-return ($prefix is null ? array<string, list<string>> : list<string>)
      */
     public function getNamespace(?string $prefix = null)
     {
diff --git a/system/Autoloader/FileLocator.php b/system/Autoloader/FileLocator.php
index 37a13e159557..f176bd6c0d7e 100644
--- a/system/Autoloader/FileLocator.php
+++ b/system/Autoloader/FileLocator.php
@@ -175,6 +175,8 @@ public function getClassname(string $file): string
      *      'app/Modules/foo/Config/Routes.php',
      *      'app/Modules/bar/Config/Routes.php',
      *  ]
+     *
+     * @return list<string>
      */
     public function search(string $path, string $ext = 'php', bool $prioritizeApp = true): array
     {
@@ -203,7 +205,7 @@ public function search(string $path, string $ext = 'php', bool $prioritizeApp =
         }
 
         // Remove any duplicates
-        return array_unique($foundPaths);
+        return array_values(array_unique($foundPaths));
     }
 
     /**
@@ -237,7 +239,7 @@ protected function getNamespaces()
         foreach ($this->autoloader->getNamespace() as $prefix => $paths) {
             foreach ($paths as $path) {
                 if ($prefix === 'CodeIgniter') {
-                    $system = [
+                    $system[] = [
                         'prefix' => $prefix,
                         'path'   => rtrim($path, '\\/') . DIRECTORY_SEPARATOR,
                     ];
@@ -252,9 +254,7 @@ protected function getNamespaces()
             }
         }
 
-        $namespaces[] = $system;
-
-        return $namespaces;
+        return array_merge($namespaces, $system);
     }
 
     /**
diff --git a/system/Commands/Generators/TestGenerator.php b/system/Commands/Generators/TestGenerator.php
index d09ecda5ae83..60863886e121 100644
--- a/system/Commands/Generators/TestGenerator.php
+++ b/system/Commands/Generators/TestGenerator.php
@@ -14,7 +14,9 @@
 namespace CodeIgniter\Commands\Generators;
 
 use CodeIgniter\CLI\BaseCommand;
+use CodeIgniter\CLI\CLI;
 use CodeIgniter\CLI\GeneratorTrait;
+use Config\Services;
 
 /**
  * Generates a skeleton command file.
@@ -66,7 +68,8 @@ class TestGenerator extends BaseCommand
      * @var array<string, string>
      */
     protected $options = [
-        '--force' => 'Force overwrite existing file.',
+        '--namespace' => 'Set root namespace. Default: "APP_NAMESPACE".',
+        '--force'     => 'Force overwrite existing file.',
     ];
 
     /**
@@ -76,9 +79,70 @@ public function run(array $params)
     {
         $this->component = 'Test';
         $this->template  = 'test.tpl.php';
-        $this->namespace = 'Tests';
 
         $this->classNameLang = 'CLI.generator.className.test';
         $this->generateClass($params);
     }
+
+    /**
+     * Builds the test file path from the class name.
+     *
+     * @param string $class namespaced classname or namespaced view.
+     */
+    protected function buildPath(string $class): string
+    {
+        $namespace = $this->getNamespace();
+
+        $base = $this->searchTestFilePath($namespace);
+
+        if ($base === null) {
+            CLI::error(
+                lang('CLI.namespaceNotDefined', [$namespace]),
+                'light_gray',
+                'red'
+            );
+            CLI::newLine();
+
+            return '';
+        }
+
+        $realpath = realpath($base);
+        $base     = ($realpath !== false) ? $realpath : $base;
+
+        $file = $base . DIRECTORY_SEPARATOR
+            . str_replace(
+                '\\',
+                DIRECTORY_SEPARATOR,
+                trim(str_replace($namespace . '\\', '', $class), '\\')
+            ) . '.php';
+
+        return implode(
+            DIRECTORY_SEPARATOR,
+            array_slice(
+                explode(DIRECTORY_SEPARATOR, $file),
+                0,
+                -1
+            )
+        ) . DIRECTORY_SEPARATOR . $this->basename($file);
+    }
+
+    /**
+     * Returns test file path for the namespace.
+     */
+    private function searchTestFilePath(string $namespace): ?string
+    {
+        $bases = Services::autoloader()->getNamespace($namespace);
+
+        $base = null;
+
+        foreach ($bases as $candidate) {
+            if (str_contains($candidate, '/tests/')) {
+                $base = $candidate;
+
+                break;
+            }
+        }
+
+        return $base;
+    }
 }
diff --git a/system/Config/AutoloadConfig.php b/system/Config/AutoloadConfig.php
index f70d07155613..3866ce760a56 100644
--- a/system/Config/AutoloadConfig.php
+++ b/system/Config/AutoloadConfig.php
@@ -91,7 +91,7 @@ class AutoloadConfig
      * @var array<string, string>
      */
     protected $corePsr4 = [
-        'CodeIgniter' => SYSTEMPATH,
+        'CodeIgniter' => [SYSTEMPATH, TESTPATH . 'system'],
         'Config'      => APPPATH . 'Config',
         'Tests'       => ROOTPATH . 'tests',
     ];
@@ -106,7 +106,7 @@ class AutoloadConfig
      * searched for within one or more directories as they would if they
      * were being autoloaded through a namespace.
      *
-     * @var array<string, string>
+     * @var array<class-string, string>
      */
     protected $coreClassmap = [
         AbstractLogger::class                  => SYSTEMPATH . 'ThirdParty/PSR/Log/AbstractLogger.php',
diff --git a/tests/system/Autoloader/AutoloaderTest.php b/tests/system/Autoloader/AutoloaderTest.php
index fbefbe20b0ef..6a1b496b05c9 100644
--- a/tests/system/Autoloader/AutoloaderTest.php
+++ b/tests/system/Autoloader/AutoloaderTest.php
@@ -97,13 +97,13 @@ public function testInitializeTwice(): void
         $loader->initialize(new Autoload(), new Modules());
 
         $ns = $loader->getNamespace();
-        $this->assertCount(1, $ns['App']);
+        $this->assertCount(2, $ns['App']);
         $this->assertSame('ROOTPATH/app', clean_path($ns['App'][0]));
 
         $loader->initialize(new Autoload(), new Modules());
 
         $ns = $loader->getNamespace();
-        $this->assertCount(1, $ns['App']);
+        $this->assertCount(2, $ns['App']);
         $this->assertSame('ROOTPATH/app', clean_path($ns['App'][0]));
     }
 
diff --git a/tests/system/Commands/TestGeneratorTest.php b/tests/system/Commands/TestGeneratorTest.php
index 8ffe89c3de55..888d8b03d1a1 100644
--- a/tests/system/Commands/TestGeneratorTest.php
+++ b/tests/system/Commands/TestGeneratorTest.php
@@ -41,6 +41,6 @@ protected function tearDown(): void
     public function testGenerateTest(): void
     {
         command('make:test Foo/Bar');
-        $this->assertFileExists(ROOTPATH . 'tests/Foo/Bar.php');
+        $this->assertFileExists(ROOTPATH . 'tests/app/Foo/Bar.php');
     }
 }
diff --git a/user_guide_src/source/cli/cli_generators.rst b/user_guide_src/source/cli/cli_generators.rst
index 2f8d7c9d55bf..ff539b3c848d 100644
--- a/user_guide_src/source/cli/cli_generators.rst
+++ b/user_guide_src/source/cli/cli_generators.rst
@@ -248,6 +248,7 @@ Argument:
 
 Options:
 ========
+* ``--namespace``: Set the root namespace. Defaults to value of ``APP_NAMESPACE``.
 * ``--force``: Set this flag to overwrite existing files on destination.
 
 make:migration