HMS Upstart (P65) was a Royal Navy U-class submarine built by Vickers-Armstrong.

After the war, she was loaned to the Greek Navy and renamed Amfitriti.

Upstart spent most of her wartime career operating off the south coast of France, where she sank the French fishing vessels Grotte de Bethlehem and Torpille, the German auxiliary minelayer Niedersachsen (the former French Guyane) and the German merchant Tolentino (the former French Saumur).

Upstart survived the war and was loaned to the Greek Navy in 1945, where she was renamed Amfitriti.

She was subsequently sunk as an ASDIC target off the Isle of Wight on 29 July 1959.