Montanes won the first four games before play was suspended by rain and then, after play resumed, went on to post a 6-0, 7-6 (7-3) victory and pick up some momentum heading into next week's French Open.
It was the first tournament title for Montanes since 2010 and the sixth of his career. Monfils fell to 4-14 in championship matches.
Monfils shook off the shutout in the first set and gave Montanes a battle in the second. Montanes held a 3-1 lead in the second set but Monfils broke serve in the seventh game and forced a tiebreak.