England's Paul Casey produced a bogey-free, six-under-par 66 in the final round of the European Open to win by one stroke in Hamburg Sunday.
Ritthammer and MacIntyre had an opportunity to force a playoff, but both left their birdie putts short on the final green as Casey became the third successive Englishman to win the event after Jordan Smith and Richard McEvoy.
It was a second victory this year for the 42-year-old, who lifted the trophy at the Valspar Championship on the U.S. PGA Tour earlier in the season, and Casey's first win in Europe since the 2014 KLM Open in the Netherlands.
"This is an incredibly prestigious tournament," said Casey, who worked at the European Open as a 14-year-old volunteer in 1991. "I'm over the moon to be European Open champion.
"It was nice to make the putts when it counted. And that really was the key. The ball-striking was excellent. I gave myself an awful lot of chances."
Scottish rookie MacIntyre, who finished sixth in the British Open, continued his impressive run with a closing 68 as he secured a runner-up finish for the third time this year.
The 23-year-old shared second spot with German Ritthammer and Matthias Schwab of Austria.
Austrian Bernd Wiesberger finished fifth at 12-under after a superb 64, while playing partner Romain Wattel of France was one shot further back in sixth. Italian Guido Migliozzi and Spain's Pablo Larrazabal finished joint seventh on 10-under.