William Stemmyer (May 6, 1865 – May 3, 1945) was a Major League Baseball pitcher for the Boston Beaneaters and Cleveland Blues. He was nicknamed "Cannonball".
In 1886 – his first and only full season in the majors – Stemmyer led the National League in strikeouts per 9 innings (6.17) and wild pitches (63). The 63 wild pitches is still the highest single-season total in National League history.
Stemmyer died in his hometown three days before his 80th birthday.