The win was Cilic's fourth at the ATP 250 tournament, following up his triumphs in 2009 and 2010 as well as last year, and the 10th Tour success of his career.
It also marked Cilic's first tournament win since a suspension for doping last year.
Cilic thundered 10 aces and won 80 per cent of points on his first serve to blow Haas away in one hour and 19 minutes.
A sloppy start to the match saw both men broken in their opening service games, before Cilic grabbed a decisive break to move 4-2 ahead en route to winning the first set.
The home favourite saved a break point in the sixth game of the next set, before breaking through again on the Haas serve.
Haas was able to save one match point, but Cilic soon wrapped up a comfortable victory, taking his record in Zagreb to 20 wins from his last 21 games.