World number seven Thiem, the fastest-rising player on the ATP Tour, won on clay in Buenos Aires and Nice, and on hard courts in Acapulco this year.
The 22-year-old's victory, his 45th of the year, also took him above Novak Djokovic to the top of the match-wins list for 2016.
Kohlschreiber was quick out of the blocks on Monday, snatching the tie break to move ahead.
Thiem, who knocked out Roger Federer in the semi-finals, responded with an early and decisive break in the second set and needed just one break in the decider to claim victory.
(Reporting by Martyn Herman; Editing by John O'Brien)
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