HMS Unrivalled (P45) was a U-class submarine built for the Royal Navy during World War II. So far she has been the only ship of the Royal Navy to bear the name Unrivalled. Completed in 1942, the boat spent most of the war in the Mediterranean. She sank a number of small merchant ships and naval auxiliaries, but major success eluded her during the war. Too small and slow for the post-war environment, Unrivalled was scrapped in 1946.