HMS Ruler was the lead ship of her class of escort carrier of the Royal Navy during the Second World War. She was built in the United States as the Bogue-class carrier St. Joseph (AVG/CVE/ACV-50) for Lend-Lease to the United Kingdom.

The Ruler-class of escort aircraft carriers served with the Royal Navy during the Second World War. All twenty-three ships were built by the Seattle-Tacoma Shipbuilding Corporation in the United States as Bogue-class escort carriers, supplied under Lend-Lease to the United Kingdom. They were the most numerous single class of aircraft carriers in service with the Royal Navy.