HMS Upholder (P37) was a Royal Navy U-class submarine built by Vickers-Armstrong at Barrow-in-Furness. She was laid down on 30 October 1939, launched on 8 July 1940 by Mrs. Doris Thompson, wife of a director of the builders. The submarine was commissioned on 31 October 1940. She was one of four U-class submarines which had two external torpedo tubes at the bows in addition to the 4 internal ones fitted to all boats. They were excluded from the others because they interfered with depth-keeping at periscope depth.

The British U-class submarines were a class of 49 small submarines built just before and during the Second World War. The class is sometimes known as the Undine class, after the first submarine built. A further development was the British V-class submarine of 1942.