The Croatian crowd favorite swept past the top-seeded Haas 6-3, 6-4 in 80 minutes, adding the 2014 title to his previous victories at the event in 2013, 2010 and 2009.
Cilic, seeded fifth, lost serve in the match's first game, but then rebounded to win five of the next six games to gain the upper hand.
Germany's Haas was leading 3-2 in the second set, but Cilic saved a break point in the sixth game and went on to win three straight games for a 5-3 advantage en route to the victory.
Haas fell to 15-13 all-time in finals.
2014: NFL Cheerleaders [PHOTOS]
Michael Sam cut from the Dallas Cowboys