William Johnston Clymer (December 18, 1873 – December 26, 1936) nicknamed "Derby Day Bill", was an American professional baseball player and manager. He played in Major League Baseball as a shortstop in 1891. After his playing career, Clymer became a successful minor league manager.

At the age of 17, Clymer appeared in three games for the Philadelphia Athletics of the American Association.^[1] In 11 at-bats as a Major League player, he did not collect a hit, but did have one base on balls and one stolen base.^[2]

Clymer went on to play 18 seasons in the minor leagues (1891–1906)^[3] and was a minor league manager for approximately 29 years (1898–1932).^[1] He was the second minor league manager with more than 2,000 wins.^[1] Clymer died at the age of 63 in his hometown of Philadelphia, and is interred at North Cedar Hill Cemetery.^[2]