diff --git a/tests/phpunit/tests/admin/includes/misc/saveModRewriteRules.php b/tests/phpunit/tests/admin/includes/misc/saveModRewriteRules.php
new file mode 100644
index 0000000000000..c6f26324b8052
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/misc/saveModRewriteRules.php
@@ -0,0 +1,268 @@
+<?php
+
+/**
+ * Tests for the save_mod_rewrite_rules() function.
+ *
+ * @group admin
+ * @group rewrite
+ *
+ * @covers ::save_mod_rewrite_rules
+ */
+class Tests_save_mod_rewrite_rules extends WP_UnitTestCase {
+
+	/**
+	 * Original home path.
+	 * @var string|bool
+	 */
+	private $orig_home;
+
+	/**
+	 * Original siteurl path.
+	 * @var string|bool
+	 */
+	private $orig_siteurl;
+
+	/**
+	 * Temporary home directory.
+	 * @var string
+	 */
+	private $temp_home;
+
+	/**
+	 * Original $_SERVER['SCRIPT_FILENAME']
+	 * @var string|null
+	 */
+	private $orig_script_filename;
+
+	/**
+	 * Original $_SERVER['SERVER_SOFTWARE']
+	 * @var string|null
+	 */
+	private $orig_server_software;
+
+	public function set_up() {
+		parent::set_up();
+
+		// Multisite test should be handled in test method if possible or skipped if globally multisite.
+		if ( is_multisite() ) {
+			// This test class is not intended for multisite.
+			return;
+		}
+
+		// Ensure the function is available.
+		require_once ABSPATH . 'wp-admin/includes/misc.php';
+
+		$this->orig_home            = get_option( 'home' );
+		$this->orig_siteurl         = get_option( 'siteurl' );
+		$this->orig_script_filename = isset( $_SERVER['SCRIPT_FILENAME'] ) ? $_SERVER['SCRIPT_FILENAME'] : null;
+		$this->orig_server_software = isset( $_SERVER['SERVER_SOFTWARE'] ) ? $_SERVER['SERVER_SOFTWARE'] : null;
+
+		$this->temp_home = get_temp_dir() . 'wp_test_home/';
+		if ( ! is_dir( $this->temp_home ) ) {
+			mkdir( $this->temp_home );
+		}
+	}
+
+	public function tear_down() {
+		if ( $this->temp_home && is_dir( $this->temp_home ) ) {
+			$this->recursive_delete( $this->temp_home );
+		}
+
+		update_option( 'home', $this->orig_home );
+		update_option( 'siteurl', $this->orig_siteurl );
+		if ( null !== $this->orig_script_filename ) {
+			$_SERVER['SCRIPT_FILENAME'] = $this->orig_script_filename;
+		} else {
+			unset( $_SERVER['SCRIPT_FILENAME'] );
+		}
+		if ( null !== $this->orig_server_software ) {
+			$_SERVER['SERVER_SOFTWARE'] = $this->orig_server_software;
+		} else {
+			unset( $_SERVER['SERVER_SOFTWARE'] );
+		}
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Helper to delete a directory and its contents.
+	 *
+	 * @param string $dir Path to the directory.
+	 */
+	private function recursive_delete( $dir ) {
+		foreach ( scandir( $dir ) as $file ) {
+			if ( '.' === $file || '..' === $file ) {
+				continue;
+			}
+			$path = $dir . DIRECTORY_SEPARATOR . $file;
+			if ( is_dir( $path ) ) {
+				$this->recursive_delete( $path );
+			} else {
+				unlink( $path );
+			}
+		}
+		rmdir( $dir );
+	}
+
+	/**
+	 * Tests that save_mod_rewrite_rules() correctly writes rules to .htaccess.
+	 *
+	 * @ticket 65146
+	 */
+	public function test_save_mod_rewrite_rules_success() {
+		global $wp_rewrite;
+
+		// Setup temporary home.
+		$home_url = 'http://example.org';
+		$site_url = 'http://example.org/wp';
+		update_option( 'home', $home_url );
+		update_option( 'siteurl', $site_url );
+
+		$_SERVER['SCRIPT_FILENAME'] = $this->temp_home . 'wp/index.php';
+		if ( ! is_dir( $this->temp_home . 'wp' ) ) {
+			mkdir( $this->temp_home . 'wp' );
+		}
+
+		$_SERVER['SERVER_SOFTWARE'] = 'Apache';
+
+		// Mock got_mod_rewrite().
+		add_filter( 'got_rewrite', '__return_true' );
+
+		// Mock WP_Rewrite rules.
+		$orig_structure = $wp_rewrite->permalink_structure;
+		$wp_rewrite->set_permalink_structure( '/%postname%/' );
+
+		$htaccess_file = $this->temp_home . '.htaccess';
+		touch( $htaccess_file );
+
+		$result = save_mod_rewrite_rules();
+
+		// Cleanup structure.
+		$wp_rewrite->set_permalink_structure( $orig_structure );
+		remove_filter( 'got_rewrite', '__return_true' );
+
+		$this->assertTrue( $result, 'save_mod_rewrite_rules() should return true on success.' );
+		$this->assertFileExists( $htaccess_file );
+
+		$content = file_get_contents( $htaccess_file );
+		$this->assertStringContainsString( '# BEGIN WordPress', $content );
+		$this->assertStringContainsString( 'RewriteEngine On', $content );
+	}
+
+	/**
+	 * Tests that save_mod_rewrite_rules() returns false if mod_rewrite is not available.
+	 *
+	 * @ticket 65146
+	 */
+	public function test_save_mod_rewrite_rules_no_mod_rewrite() {
+		// Mock got_mod_rewrite() as false.
+		add_filter( 'got_rewrite', '__return_false' );
+
+		// Setup path.
+		$home_url = 'http://example.org';
+		$site_url = 'http://example.org/wp';
+		update_option( 'home', $home_url );
+		update_option( 'siteurl', $site_url );
+		$_SERVER['SCRIPT_FILENAME'] = $this->temp_home . 'wp/index.php';
+		if ( ! is_dir( $this->temp_home . 'wp' ) ) {
+			mkdir( $this->temp_home . 'wp' );
+		}
+
+		touch( $this->temp_home . '.htaccess' );
+
+		$_SERVER['SERVER_SOFTWARE'] = 'Apache';
+
+		$result = save_mod_rewrite_rules();
+
+		remove_filter( 'got_rewrite', '__return_false' );
+
+		$this->assertFalse( $result, 'save_mod_rewrite_rules() should return false when mod_rewrite is not available.' );
+	}
+	/**
+	 * Tests that save_mod_rewrite_rules() returns false if the .htaccess file is not writable.
+	 *
+	 * @ticket 65146
+	 */
+	public function test_save_mod_rewrite_rules_non_writable() {
+		// Mock is_writable to return false.
+		// Since we can't easily mock is_writable, we try the chmod approach.
+		// If it is skipped, we'll know.
+		if ( 'root' === posix_getpwuid( posix_geteuid() )['name'] ) {
+			$this->markTestSkipped( 'Cannot test non-writable files as root.' );
+		}
+
+		// Setup path.
+		$home_url = 'http://example.org';
+		$site_url = 'http://example.org/wp';
+		update_option( 'home', $home_url );
+		update_option( 'siteurl', $site_url );
+		$_SERVER['SCRIPT_FILENAME'] = $this->temp_home . 'wp/index.php';
+		if ( ! is_dir( $this->temp_home . 'wp' ) ) {
+			mkdir( $this->temp_home . 'wp' );
+		}
+
+		$htaccess_file = $this->temp_home . '.htaccess';
+		touch( $htaccess_file );
+		chmod( $htaccess_file, 0444 ); // Read-only.
+
+		$_SERVER['SERVER_SOFTWARE'] = 'Apache';
+
+		// Mock got_mod_rewrite().
+		add_filter( 'got_rewrite', '__return_true' );
+
+		$result = save_mod_rewrite_rules();
+
+		chmod( $htaccess_file, 0666 ); // Restore for cleanup.
+		remove_filter( 'got_rewrite', '__return_true' );
+
+		$this->assertFalse( $result, 'save_mod_rewrite_rules() should return false when .htaccess is not writable.' );
+	}
+
+	/**
+	 * Tests that save_mod_rewrite_rules() returns false if the directory is not writable and file doesn't exist.
+	 *
+	 * @ticket 65146
+	 */
+	public function test_save_mod_rewrite_rules_non_writable_dir() {
+		if ( 'root' === posix_getpwuid( posix_geteuid() )['name'] ) {
+			$this->markTestSkipped( 'Cannot test non-writable directories as root.' );
+		}
+
+		$home_dir = $this->temp_home . 'non_writable_home/';
+		mkdir( $home_dir );
+
+		// Setup path.
+		$home_url = 'http://example.org';
+		$site_url = 'http://example.org/wp';
+		update_option( 'home', $home_url );
+		update_option( 'siteurl', $site_url );
+		$_SERVER['SCRIPT_FILENAME'] = $home_dir . 'wp/index.php';
+		mkdir( $home_dir . 'wp' );
+
+		chmod( $home_dir, 0555 ); // Read-only dir.
+
+		$_SERVER['SERVER_SOFTWARE'] = 'Apache';
+
+		// Mock got_mod_rewrite().
+		add_filter( 'got_rewrite', '__return_true' );
+
+		$result = save_mod_rewrite_rules();
+
+		chmod( $home_dir, 0777 ); // Restore for cleanup.
+		remove_filter( 'got_rewrite', '__return_true' );
+
+		$this->assertFalse( $result, 'save_mod_rewrite_rules() should return false when home directory is not writable.' );
+	}
+	/**
+	 * Tests that save_mod_rewrite_rules() returns null in multisite.
+	 *
+	 * @ticket 65146
+	 */
+	public function test_save_mod_rewrite_rules_multisite() {
+		if ( ! is_multisite() ) {
+			$this->markTestSkipped( 'This test requires multisite.' );
+		}
+
+		$this->assertNull( save_mod_rewrite_rules() );
+	}
+}