diff --git a/tests/phpunit/tests/functions/getTempDir.php b/tests/phpunit/tests/functions/getTempDir.php
new file mode 100644
index 0000000000000..2e660985bcfb2
--- /dev/null
+++ b/tests/phpunit/tests/functions/getTempDir.php
@@ -0,0 +1,53 @@
+<?php
+
+/**
+ * Tests the get_temp_dir() function.
+ *
+ * @group functions
+ *
+ * @covers ::get_temp_dir
+ */
+class Tests_Functions_GetTempDir extends WP_UnitTestCase {
+
+	/**
+	 * Tests that get_temp_dir() returns a path with a trailing slash.
+	 *
+	 * @ticket 65651
+	 */
+	public function test_get_temp_dir_has_trailing_slash() {
+		$temp_dir = get_temp_dir();
+		$this->assertStringEndsWith( '/', $temp_dir );
+	}
+
+	/**
+	 * Tests that get_temp_dir() returns a writable directory.
+	 *
+	 * @ticket 65651
+	 */
+	public function test_get_temp_dir_is_writable_directory() {
+		$temp_dir = get_temp_dir();
+		$this->assertDirectoryExists( $temp_dir );
+		$this->assertTrue( wp_is_writable( $temp_dir ), 'The temporary directory must be writable.' );
+	}
+
+	/**
+	 * Tests that get_temp_dir() respects the WP_TEMP_DIR constant.
+	 *
+	 * Since constants cannot be redefined, we use a separate process to test this.
+	 *
+	 * @ticket 65651
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_get_temp_dir_respects_constant() {
+		$custom_temp = '/tmp/wp-custom-temp/';
+		if ( 'Windows' === PHP_OS_FAMILY ) {
+			$custom_temp = 'C:\\Windows\\Temp\\wp-custom-temp\\';
+		}
+
+		define( 'WP_TEMP_DIR', $custom_temp );
+
+		$this->assertSame( trailingslashit( $custom_temp ), get_temp_dir() );
+	}
+}