diff --git a/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb b/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb
new file mode 100644
index 000000000..2d53227ed
--- /dev/null
+++ b/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb
@@ -0,0 +1,14 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+class Issue3618GoogleCalendarUrlHttps < ActiveRecord::Migration[5.2]
+  def change
+    return if !Setting.exists?(name: 'system_init_done')
+
+    Calendar
+      .where('ical_url LIKE ?', 'http://www.google.com/calendar/ical/%')
+      .each do |calendar|
+        calendar.ical_url.sub!(%r{^http://}, 'https://')
+        calendar.save
+      end
+  end
+end
diff --git a/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb b/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb
new file mode 100644
index 000000000..05f526855
--- /dev/null
+++ b/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb
@@ -0,0 +1,15 @@
+# Copyright (C) 2012-2021 Zammad Foundation, http://zammad-foundation.org/
+
+require 'rails_helper'
+
+RSpec.describe Issue3618GoogleCalendarUrlHttps, type: :db_migration, db_strategy: :reset do
+  let(:url)      { 'http://www.google.com/calendar/ical/en.lithuanian%%23holiday%%40group.v.calendar.google.com/public/basic.ics' }
+  let(:calendar) { create :calendar, ical_url: url }
+
+  it 'migrates Google Calendar URLs' do
+    expect { migrate }
+      .to change { calendar.reload.ical_url.starts_with? 'https://' }
+      .from(false)
+      .to(true)
+  end
+end