diff --git a/core/scripts/switch-psr4.sh b/core/scripts/switch-psr4.sh
new file mode 100644
index 000000000000..0eebbe5e858e
--- /dev/null
+++ b/core/scripts/switch-psr4.sh
@@ -0,0 +1,319 @@
+#!/bin/php
+<?php
+
+namespace Drupal\Core\SwitchPsr4;
+
+/**
+ * @file
+ * Moves module-provided class files to their PSR-4 location.
+ *
+ * E.g.:
+ * core/modules/action/{lib/Drupal/action → src}/ActionAccessController.php
+ * core/modules/action/{lib/Drupal/action → src}/ActionAddFormController.php
+ */
+
+// Determine DRUPAL_ROOT.
+$dir = dirname(__FILE__);
+while (!defined('DRUPAL_ROOT')) {
+  if (is_dir($dir . '/core')) {
+    define('DRUPAL_ROOT', $dir);
+  }
+  $dir = dirname($dir);
+}
+
+// Run the script.
+run();
+
+/**
+ * Runs the script.
+ */
+function run() {
+  $cmd_arguments = $_SERVER['argv'];
+  // The first argument is the script name.
+  $scriptname = array_shift($cmd_arguments);
+  if (in_array('--help', $cmd_arguments)) {
+    print get_help_text($scriptname);
+  }
+  elseif (!empty($cmd_arguments)) {
+    // If one or more arguments are given, treat those arguments as directories,
+    // and process all modules found within these directories.
+    $directories = array();
+    foreach ($cmd_arguments as $arg) {
+      if ('-' === $arg{0}) {
+        // The only valid option is '--help'.
+        print "Invalid option '$arg'";
+        return;
+      }
+      if (!is_dir($arg)) {
+        print "The argument '$arg' is not a directory.";
+        continue;
+      }
+      $directories[] = $arg;
+    }
+    // Process all directories that were found in the argument list.
+    foreach ($directories as $dir) {
+      process_candidate_dir($dir);
+    }
+  }
+  else {
+    // If no arguments are given, process all modules and profiles in the core
+    // directories instead.
+    process_candidate_dir(DRUPAL_ROOT . '/core/modules');
+    process_candidate_dir(DRUPAL_ROOT . '/core/profiles');
+  }
+}
+
+/**
+ * @param string $scriptname
+ *
+ * @return string
+ *   Help text in case the "--help" option is present.
+ */
+function get_help_text($scriptname) {
+  $script = basename($scriptname);
+  return <<<EOF
+
+Move module class files from PSR-0 to PSR-4.
+
+E.g. the following files would be moved.
+  - core/modules/action/{lib/Drupal/action → src}/ActionListController.php
+  - core/modules/action/tests/{Drupal/action/Tests → src}/Menu/ActionLocalTasksTest.php
+
+Class files which are already in the PSR-4 path remain where they are.
+
+Warning: Classes with an underscore in the class name (after the last namespace
+separator) will end up in an incorrect location, and need to be fixed manually.
+Such class names are not allowed in Drupal coding standards, but they may still
+occur in some custom and contrib modules.
+
+The script takes any number of arguments which, if present, specify the
+directories to scan for modules and module class files.
+
+If no arguments are given, the following directories will be processed:
+  - core/modules/
+  - core/profiles/
+
+See:
+  - https://drupal.org/node/2083547 Drupal issue
+  - https://drupal.org/node/2156625 Documentation of PSR-4 in Drupal
+
+Usage:        {$script} [OPTIONS] [DIRECTORIES]
+Examples:     {$script}
+              {$script} core/modules/views
+              {$script} modules/contrib modules/custom
+              {$script} modules/contrib/devel
+
+Options:
+  --help      Display this help page and exit.
+
+
+EOF;
+}
+
+/**
+ * Scans all subdirectories of a given directory for Drupal extensions, and runs
+ * process_extension() for each one that it finds.
+ *
+ * @param string $dir
+ *   A directory whose subdirectories could contain Drupal extensions.
+ */
+function process_extensions_base_dir($dir) {
+  /**
+   * @var \DirectoryIterator $fileinfo
+   */
+  foreach (new \DirectoryIterator($dir) as $fileinfo) {
+    if ($fileinfo->isDot()) {
+      // do nothing
+    }
+    elseif ($fileinfo->isDir()) {
+      process_candidate_dir($fileinfo->getPathname());
+    }
+  }
+}
+
+/**
+ * Recursively scans a directory for Drupal extensions, and runs
+ * process_extension() for each one that it finds.
+ *
+ * @param string $dir
+ *   A directory that could be a Drupal extension directory.
+ */
+function process_candidate_dir($dir) {
+  /**
+   * @var \DirectoryIterator $fileinfo
+   */
+  foreach (new \DirectoryIterator($dir) as $fileinfo) {
+    if ($fileinfo->isDot()) {
+      // Ignore "." and "..".
+    }
+    elseif ($fileinfo->isDir()) {
+      // It's a directory.
+      switch ($fileinfo->getFilename()) {
+        case 'lib':
+        case 'src':
+          // Ignore these directory names.
+          continue;
+        default:
+          // Look for more extensions in subdirectories.
+          process_candidate_dir($fileinfo->getPathname());
+      }
+    }
+    else {
+      // It's a file.
+      if (preg_match('/^(.+).info.yml$/', $fileinfo->getFilename(), $m)) {
+        // It's a *.info.yml file, so we found an extension directory.
+        $extension_name = $m[1];
+      }
+    }
+  }
+  if (isset($extension_name)) {
+    process_extension($extension_name, $dir);
+    process_extension_phpunit($extension_name, $dir);
+  }
+}
+
+/**
+ * Process a Drupal extension (module, theme) in a directory.
+ *
+ * This will move all class files in this extension from
+ * lib/Drupal/$extension_name/$path to src/$path.
+ *
+ * @param string $name
+ *   Name of the extension.
+ * @param string $dir
+ *   Directory of the extension.
+ * @throws \Exception
+ */
+function process_extension($name, $dir) {
+
+  if (!is_dir($source = "$dir/lib/Drupal/$name")) {
+    // Nothing to do in this module.
+    return;
+  }
+
+  if (!is_dir($destination = "$dir/src")) {
+    mkdir($destination);
+  }
+
+  // Move class files two levels up.
+  move_directory_contents($source, $destination);
+
+  // Clean up.
+  require_dir_empty("$dir/lib/Drupal");
+  rmdir("$dir/lib/Drupal");
+}
+
+/**
+ * Process a Drupal extension (module, theme) in a directory.
+ *
+ * This will move all PHPUnit class files in this extension from
+ * tests/Drupal/$name/Tests/ to tests/src/.
+ *
+ * @param string $name
+ *   Name of the extension.
+ * @param string $dir
+ *   Directory of the extension.
+ */
+function process_extension_phpunit($name, $dir) {
+
+  if (!is_dir($source = "$dir/tests/Drupal/$name/Tests")) {
+    // Nothing to do in this module.
+    return;
+  }
+
+  if (!is_dir($dest = "$dir/tests/src")) {
+    mkdir($dest);
+  }
+
+  // Move class files two levels up.
+  move_directory_contents($source, $dest);
+
+  // Clean up.
+  require_dir_empty("$dir/tests/Drupal/$name");
+  rmdir("$dir/tests/Drupal/$name");
+  require_dir_empty("$dir/tests/Drupal");
+  rmdir("$dir/tests/Drupal");
+}
+
+/**
+ * Move directory contents from an existing source directory to an existing
+ * destination directory.
+ *
+ * @param string $source
+ *   An existing source directory.
+ * @param string $destination
+ *   An existing destination directory.
+ *
+ * @throws \Exception
+ */
+function move_directory_contents($source, $destination) {
+
+  if (!is_dir($source)) {
+    throw new \Exception("The source '$source' is not a directory.");
+  }
+
+  if (!is_dir($destination)) {
+    throw new \Exception("The destination '$destination' is not a directory.");
+  }
+
+  /**
+   * @var \DirectoryIterator $fileinfo
+   */
+  foreach (new \DirectoryIterator($source) as $fileinfo) {
+    if ($fileinfo->isDot()) {
+      continue;
+    }
+    $dest_path = $destination . '/' . $fileinfo->getFilename();
+    if (!file_exists($dest_path)) {
+      rename($fileinfo->getPathname(), $dest_path);
+    }
+    elseif ($fileinfo->isFile()) {
+      throw new \Exception("Destination '$dest_path' already exists, cannot overwrite.");
+    }
+    elseif ($fileinfo->isDir()) {
+      if (!is_dir($dest_path)) {
+        throw new \Exception("Destination '$dest_path' is not a directory.");
+      }
+      move_directory_contents($fileinfo->getPathname(), $dest_path);
+    }
+  }
+
+  require_dir_empty($source);
+
+  rmdir($source);
+}
+
+/**
+ * Throws an exception if a directory is not empty.
+ *
+ * @param string $dir
+ *   Directory to check.
+ *
+ * @throws \Exception
+ */
+function require_dir_empty($dir) {
+  if (is_file($dir)) {
+    throw new \Exception("The path '$dir' is a file, when it should be a directory.");
+  }
+  if (!is_dir($dir)) {
+    throw new \Exception("The directory '$dir' does not exist.");
+  }
+  if (!is_readable($dir)) {
+    throw new \Exception("The directory '$dir' is not readable.");
+  }
+  /**
+   * @var \DirectoryIterator $fileinfo
+   */
+  foreach (new \DirectoryIterator($dir) as $fileinfo) {
+    if ($fileinfo->isDot()) {
+      continue;
+    }
+    $path = $fileinfo->getPathname();
+    if ($fileinfo->isFile()) {
+      throw new \Exception("File '$path' found in a directory that should be empty.");
+    }
+    elseif ($fileinfo->isDir()) {
+      throw new \Exception("Subdirectory '$path' found in a directory that should be empty.");
+    }
+  }
+}