The Argentine third seed, 29, needed just over an hour and 11 minutes to see off the Finnish world number 42, who will rise back into the top 40 after reaching the final of the first Power Horse Cup in the German city.
Monaco, the world number 19, served well, landing four aces, never double faulting and winning over 70% of his points on both first and second serve.
The Argentine has not dropped a set this week and was broken just the once, in the opening set, and took all four of his own break points, while saving two.
It is Monaco's eighth career title and gives him a 9-9 record in finals.
The unfortunate Nieminen, 31, has now lost nine out of his 11 career ATP finals.