diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 64dce4a159755..51dc18d52e5e5 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -2093,7 +2093,7 @@ function wp_mkdir_p( $target ) {
 		$dir_perms = 0777;
 	}
 
-	if ( @mkdir( $target, $dir_perms, true ) ) {
+	if ( mkdir( $target, $dir_perms, true ) || is_dir( $target ) ) {
 
 		/*
 		 * If a umask is set that modifies $dir_perms, we'll have to re-set
diff --git a/tests/phpunit/tests/functions/wpMkdirP.php b/tests/phpunit/tests/functions/wpMkdirP.php
new file mode 100644
index 0000000000000..29bb56b1df906
--- /dev/null
+++ b/tests/phpunit/tests/functions/wpMkdirP.php
@@ -0,0 +1,51 @@
+<?php
+
+/**
+ * Test wp_mkdir_p().
+ *
+ * @group functions.php
+ * @covers ::wp_mkdir_p
+ */
+class Tests_Functions_wpMkdirP extends WP_UnitTestCase {
+
+	public function test_folder_is_made() {
+
+		$upload_dir = wp_upload_dir();
+		$target     = $upload_dir['basedir'] . 'test';
+		$this->assertTrue( wp_mkdir_p( $target ) );
+		$this->assertDirectoryExists( $target );
+	}
+
+	public function test_if_file_with_name_extists() {
+
+		$this->assertFalse( wp_mkdir_p( ABSPATH . 'wp-admin/index.php' ) );
+	}
+
+	public function test_if_folder_with_name_exists() {
+
+		$this->assertTrue( wp_mkdir_p( ABSPATH . 'wp-admin' ) );
+	}
+
+	// should return false ../ in path
+	// Do not allow path traversals.
+	public function test_if_up_tree_in_target() {
+
+		$this->assertFalse( wp_mkdir_p( '../wp-admin' ) );
+		$this->assertFalse( wp_mkdir_p( '../../test' ) );
+		$this->assertFalse( wp_mkdir_p( '..' . DIRECTORY_SEPARATOR . 'test' ) );
+		$this->assertFalse( wp_mkdir_p( ABSPATH . 'test/../../' ) );
+
+		// this resolves to current dir so is found
+		$this->assertTrue( wp_mkdir_p( '../../../../' ) );
+	}
+
+	public function test_permissions_are_set() {
+
+		$upload_dir  = wp_upload_dir();
+		$target      = $upload_dir['basedir'] . 'permission_test';
+		$parent_stat = stat( $upload_dir['basedir'] );
+		$this->assertTrue( wp_mkdir_p( $target ) );
+		$target_stat = stat( $target );
+		$this->assertSame( $parent_stat['mode'], $target_stat['mode'] );
+	}
+}