HMS Ultimatum (P34) was a Royal Navy U-class submarine built by Vickers-Armstrong at Barrow-in-Furness, and part of the third group of that class.

However U-431 was sunk with all hands on 21 October 1943 in the Mediterranean off Algiers by depth charges from a British Wellington aircraft.

U-431's fate was revised in November 1987 by the Foreign Documents Section of the British Ministry of Defence.

Ultimatum also unsuccessfully attacked the Italian merchant ships Ravello, Luciano Manara and Unione, the Italian submarine Zaffiro and the German auxiliary patrol vessel Uj 6073 / Nimeth Allah.

She had more luck on 2 May 1944, when she shelled the harbour of Kalamata, Greece, sinking two sailing vessels, destroying five on the slips and damaging another.