Irène "Iet" Maria Jo Arnoldina van Feggelen (20 August 1921 – 17 July 2012) was a Dutch backstroke swimmer. Her first name is sometimes misspelled as Let, and her last name is often given as Koster–van Feggelen or Koster due to her marriage to the Dutch water polo player Ko Koster in 1943. Her career was hampered by World War II, yet she won two medals at the 1938 and 1947 European Aquatics Championships and set eleven world records between 1938 and 1947. After retirement, she became a national coach in conventional and synchronized swimming. In 2009, she was inducted to the International Swimming Hall of Fame.