HMS Unswerving (P63) was a Royal Navy U-class submarine built by Vickers-Armstrongs, High Walker.

So far, she has been the only ship of the Royal Navy to bear the name Unswerving.

She eventually spent most of her wartime career in the Mediterranean, where she sank the German guard boats GN 61 and GN 62, the German tanker Bertha (the former French Bacchus) and six sailing vessels, and claimed to have damaged a seventh.

Under the command of Lieutenant M. D. "Mick" Tattershall, Unswerving was the first British submarine where all the officers were members of the Royal Naval Volunteer Reserve rather than any of them being officers of the regular navy.

Unswerving survived the war and arrived at John Cashmore Ltd, Newport on 10 July 1949 for scrapping.