The Bulldogs (8-4) won their fourth in a row and advanced to Saturday's tournament championship game against Washington State. The loss snapped a five-game winning streak for the Seminoles (10-3).
Howard grabbed nine rebounds and Mack had five assists for Butler, which also got 11 points from Chase Stigall.
Butler led by as many as 15 points with 8:44 left, but Florida State came back and was within four in the final minute. Howard stole the ball and hit a pair of free throws to make it 67-61.
Chris Singleton, who hit a 3-pointer as time expired, scored 26 of his game-high 28 points in the second half for Florida State.