Monfils survives scare for Stockholm title
France's world number 10, now aged 25, has four ATP Tour titles in his career after picking up his first this season.
"My concentration was the key for me today," Monfils said. "I just believed I could win this match."
World number 73 and one-time number 13 Nieminen - whose only Tour title came in Auckland six years ago - has now lost all three of his final appearances in Stockholm.
"Even though the first set was really tight, I felt I had to be more aggressive as it was working well this week," Nieminen, 30, said.
"That's when I play my best and I played a really good second set. I continued that for the beginning of the third set, but unfortunately went down a break and got a bit tired and Monfils stayed strong at the end."