William John Petrick III (born April 29, 1984) is an American former professional baseball pitcher.

Petrick made his major league debut on June 27, 2007 for the Chicago Cubs, against the Colorado Rockies giving up two earned runs in two innings.

On February 13, 2009, Petrick signed with the Windy City ThunderBolts for their 2009 season.

Petrick signed with the Amarillo Thunderheads of the American Association of Independent Professional Baseball for the 2015 season.

Petrick's brother, Zach, is also a professional baseball player.