diff --git a/classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php b/classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php
new file mode 100644
index 00000000000..71aafee5f77
--- /dev/null
+++ b/classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php
@@ -0,0 +1,71 @@
+<?php
+
+/**
+ * @file classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php
+ *
+ * Copyright (c) 2014-2021 Simon Fraser University
+ * Copyright (c) 2000-2021 John Willinsky
+ * Distributed under the GNU GPL v3. For full terms see the file docs/COPYING.
+ *
+ * @class MergeLocalesMigration
+ *
+ * @brief Change Locales from locale_countryCode localization folder notation to locale localization folder notation
+ */
+
+namespace APP\migration\upgrade\v3_4_0;
+
+use Illuminate\Support\Collection;
+use Illuminate\Support\Facades\DB;
+use PKP\install\DowngradeNotSupportedException;
+
+class MergeLocalesMigration extends \PKP\migration\upgrade\v3_4_0\MergeLocalesMigration
+{
+    protected string $CONTEXT_TABLE = 'journals';
+    protected string $CONTEXT_SETTINGS_TABLE = 'journal_settings';
+    protected string $CONTEXT_COLUMN = 'journal_id';
+
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        parent::up();
+
+        // issue_galleys
+        $issueGalleys = DB::table('issue_galleys')
+            ->get();
+
+        foreach ($issueGalleys as $issueGalley) {
+            $this->updateSingleValueLocale($issueGalley->locale, 'issue_galleys', 'locale', 'galley_id', $issueGalley->galley_id);
+        }
+
+        // publication_galleys
+        $publicationGalleys = DB::table('publication_galleys')
+            ->get();
+
+        foreach ($publicationGalleys as $publicationGalley) {
+            $this->updateSingleValueLocale($publicationGalley->locale, 'publication_galleys', 'locale', 'galley_id', $publicationGalley->galley_id);
+        }
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        throw new DowngradeNotSupportedException();
+    }
+
+    protected function getSettingsTables(): Collection
+    {
+        return collect([
+            'issue_galley_settings',
+            'issue_settings',
+            'journal_settings',
+            'publication_galley_settings',
+            'section_settings',
+            'static_page_settings',
+            'subscription_type_settings'
+        ])->merge(parent::getSettingsTables());
+    }
+}