William Mosienko was a Canadian professional ice hockey right winger who played 14 seasons in the National Hockey League (NHL) for the Chicago Black Hawks from 1942 to 1955. He is best noted for recording the fastest hat trick in NHL history. In a 1952 game against the New York Rangers, Mosienko scored three goals in 21 seconds.

A hat-trick or hat trick is the achievement of a generally positive feat three times in a match, or another achievement based on the number three.