Ain Roost (born May 12, 1946) is a retired male discus thrower, who represented Canada twice (1972 and 1976) at the Summer Olympics. A resident of San Diego, California, he claimed the bronze medal in the men's discus throw event at the 1971 Pan American Games in Cali, Colombia and finished fifth at the 1975 Pan American Games.
Ain Roost was born in Sweden to parents who left Estonia to escape the Soviet occupation. When he was five years old, the family moved to Canada.