HMS Excalibur was an Explorer-class submarine, the sister ship of HMS Explorer, the two submarines being the only high-test peroxide (HTP) powered submarines to be constructed for the Royal Navy.

She is the only ship to be named as such, in honour of the sword of Arthurian legend.

Both Excalibur and Explorer were assigned to the 3rd Submarine Squadron although due to their experimental nature, they tended to operate independently, accompanied by their depot ship HMS Kingfisher and a fuel carrier, RFA Spabeck.

They later acted as high-speed underwater targets for the Royal Navy's prototype nuclear-powered submarine HMS Dreadnought.

This article about a specific naval submarine of the United Kingdom is a stub.