Major Derek Swithin Allhusen, CVO (9 January 1914 – 24 April 2000) was an English equestrian who was a 54-year-old grandfather when he rode Lochinvar to team gold and individual silver medals at the 1968 Summer Olympics in Mexico.

Derek Swithin Allhusen was born in London and educated at Eton and Trinity College, Cambridge.

In 1961 he bought Irish-bred Lochinvar and rode her in two winning European Championship teams (in 1967 and 1969) as well as the gold and silver at the 1968 Summer Olympics.

He was offered appointment as an MBE for his achievements but declined it; feeling his team-mates Richard Meade, Jane Bullen and Reuben Jones also deserved recognition.

On his retirement from the sport he continued as a breeder and Laurien's son Laurieston was ridden to team and individual Olympic gold medals in 1972 Games in Munich, with Richard Meade in the saddle.