William John Koski (February 6, 1932 – July 12, 2014) was an American pitcher in Major League Baseball who played for the Pittsburgh Pirates. Listed at 6' 4" (1.93 m), 185 lb. (84 kg), he batted and threw right handed. In his only starting assignment, Koski lasted 4⅓ innings against the New York Giants at Polo Grounds, giving up three earned runs on four hits and six walks, striking out two of the 23 batters he faced. The Giants won, 8–3, as he was credited with the loss. Koski died in 2014 at the age of 82.