It was the 10th-ranked Monfils' fourth career ATP Tour crown. The 25-year-old Frenchman had been hampered by a knee injury.
"After Bangkok I haven't practiced that much, mostly been in rehab for my knee," he told local news agency TT after the game. "The key today was that I managed to keep up my concentration."
Both players appeared nervous at the start of the match. Nieminen became the first player to hold serve in the fifth game.
Nieminen, who played his third Stockholm Open final at the Royal Tennis Hall, played well in the second set but Monfils rolled through the third.