Admiral Sir St. John Reginald Joseph Tyrwhitt, 2nd Baronet, KCB, DSO, DSC & Bar (18 April 1905 – 10 October 1961) was a senior Royal Navy officer who served as Second Sea Lord and Chief of Naval Personnel from 1959 to 1961.

Born the son of Admiral of the Fleet Sir Reginald Tyrwhitt and Angela Mary Corbally,^[1] Tyrwhitt joined the Royal Navy in 1919.^[2] He served in the Second World War as Commanding Officer of the destroyer HMS Defender from 1939 and then as Commander of the destroyer HMS Juno from 1940 until it was sunk by Italian bombers 30 nautical miles off Crete in 1941.^[3] He was given command of the destroyer HMS Tartar from 1942.^[2]

After inheriting his father's baronetcy in 1951,^[4] Tyrwhitt assumed command of the cruiser HMS Newcastle during the Korean War then became Flag Officer (Flotillas) to the Indian Navy in 1956,^[5] and chief of staff to the commander-in-chief of the Mediterranean Fleet in 1958. After promotion to vice-admiral on 3 November 1958,^[6] he was appointed Second Sea Lord and Chief of Naval Personnel in 1959.^[2] He was appointed a Knight Commander of the Order of the Bath in the 1961 New Year Honours and was promoted to full Admiral on 11 September 1961.^[7] He died at Queen Alexandra Military Hospital, London, shortly after leaving office, on 10 October 1961, aged 56.^[7]

In 1944, Tyrwhitt married Nancy Veronica Gilbey; they had two sons and a daughter.^[1]