Max John Flack (February 5, 1890 – July 31, 1975) was a Major League Baseball outfielder.

In his second year with the team, Flack's .314 batting average was fifth-best in the National League for the 1915 season.

In the third inning of Game 6, at Fenway Park, playing for the Cubs, his throwing error resulted in two Boston Red Sox runs, which provided the Bosox with the margin of victory, and a Series win, the last the Red Sox would achieve until 2004.

Flack's possible involvement in throwing that World Series has recently come to light.

[2] Flack is remembered, along with Cliff Heathcote, for being half of a unique player swap.