Clifford Douglas "Cliff" Lumsdon Jr., CM OOnt (April 13, 1931 – August 31, 1991) was a Canadian world champion marathon swimmer.
In 1949, at the age of 18, Lumsdon won the world marathon championship in Toronto, defeating 46 competitors in the annual 15-mile race at the Canadian National Exhibition.
Lumsdon would go on to win four more marathons at the CNE, including a 32-mile race along the Lake Ontario waterfront in 1955 (replacing a planned cross-lake competition).
Lumsdon was accompanied for part of the race by his fiancée, and by fellow Lakeshore swimmer Marilyn Bell, riding in a boat.
He coached his daughter, Kim Lumsdon, who was also a top marathon swimmer, and accompanied her during her swim across Lake Ontario in 1976.