In the first all-German final in Munich since 1965, Haas held off a late fightback from Kohlschreiber to follow Fabrice Santoro who won in Newport at the same age in 2008.
"It's a sensational feeling to be quite honest," Haas said on the ATP website (www.atptour.com) after winning his first title in 11 months.
"It was another one of those milestones that I hoped for, to maybe win this title one year. To have done that, this late in my career, is a big highlight for me.
"Every time you win a title, it's a reflection of putting everything together the whole week. It doesn't happen that often. You're really proud of those moments."
Haas's fourth title on home soil and 14th overall will move him up to number 13 in the world rankings on Monday.
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- Fabrice Santoro