William Henry Slack (born May 3, 1933, at Petrolia, Ontario) is a Canadian retired professional baseball pitcher, manager and coach, and a member of the Canadian Baseball Hall of Fame. Although he never played Major League Baseball, Slack spent 50 years at the minor league level, and was a longtime member of the Boston Red Sox and Atlanta Braves organizations. In his playing days, he threw right-handed, batted left-handed, stood 5 feet 10 inches (1.78 m) tall and weighed 175 pounds (79 kg).