diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index da7daca8bb..10d240b35b 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -129,6 +129,58 @@ class Tests_Functions extends WP_UnitTestCase {
 		}
 	}
 
+	/**
+	 * Tests path_join().
+	 *
+	 * @ticket 55897
+	 * @dataProvider path_join_data_provider
+	 */
+	public function test_path_join( $base, $path, $expected ) {
+		$this->assertSame( $expected, path_join( $base, $path ) );
+	}
+
+	/**
+	 * Data provider for test_path_join().
+	 *
+	 * @return string[][]
+	 */
+	public function path_join_data_provider() {
+		return array(
+			// Absolute path.
+			'absolute path should return path' => array(
+				'base'     => 'base',
+				'path'     => '/path',
+				'expected' => '/path',
+			),
+			// Non-absolute paths.
+			'join base and path'               => array(
+				'base'     => 'base',
+				'path'     => 'path',
+				'expected' => 'base/path',
+			),
+			'strip trailing slashes in base'   => array(
+				'base'     => 'base///',
+				'path'     => 'path',
+				'expected' => 'base/path',
+			),
+			'empty path'                       => array(
+				'base'     => 'base',
+				'path'     => '',
+				'expected' => 'base/',
+			),
+			'empty base'                       => array(
+				'base'     => '',
+				'path'     => 'path',
+				'expected' => '/path',
+			),
+			'empty path and base'              => array(
+				'base'     => '',
+				'path'     => '',
+				'expected' => '/',
+			),
+		);
+	}
+
 	/**
 	 * @ticket 33265
 	 * @ticket 35996