William Mercer (9 May 1905 – March 1965 (aged 59)) was an English professional rugby league footballer who played in the 1920s and 1930s. He played at representative level for England and Lancashire, and at club level for St. Helens (captain), as a wing, centre, second-row or loose forward, i.e. number 2 or 5, 3 or 4, 11 or 12, or 13.