William Foster (10 July 1890 – 17 December 1963) was an English competitive swimmer who represented Great Britain in the 1908 and 1912 Olympics. He was freestyle swimmer who won gold and bronze medals as a member of British relay teams, and set a world record in the 4x200-metre freestyle relay. At the 1908 Summer Olympics in London, he won the gold medal as part of the British 4×200-metre freestyle relay team and did not compete in any other competition. In the 400-metre freestyle final he finished fourth and in the 1500-metre freestyle event he was eliminated in the semi-finals.