Haase won his second career title in as many finals. The 42nd-ranked Dutchman took control of the match after an hour-long rain delay at 1-1 in the second set.
Kohlschreiber, who dropped to 4-3 in career finals, will move into the top 20 for the first time when ATP rankings are released Monday.
Kohlschreiber missed six break points as the opening set went with serve. The German won the tiebreaker when Haase hit his third double-fault at 6-2 down.
Haase converted his first chance on Kohlschreiber's serve in the opening game of the second set but dropped his own serve immediately afterward.
After the rain delay, Haase dominated play and used two more breaks to take the second set, helped by Kohlschreiber who hit his only double fault in the set at set point down.
Like in the opening set, Haase saved six break points in the third to close out the victory.