Mostyn Yanto Ffrench-Williams (12 October 1914 – 11 November 1963) was an English competition swimmer and backstroke swimmer who represented Great Britain at the Olympics, and swam for England at the British Empire Games, during the 1930s.

In the 1932 Summer Olympics, he finished fifth with the British team in the 4×200-metre freestyle relay event.

At the 1934 British Empire Games in London, he was a member of the English team which won the silver medal in the 4×200-yard freestyle competition.

In the 3×110-yard medley contest he won a bronze medal with the English team.

Four years later at the 1938 Empire Games in Sydney, he was part of the English team which won the gold medal in the 4×220-yard freestyle event.