The 33-year-old, who also lifted the trophy in 2010 and 2014, saved all four break points he faced and converted two out of four to secure the win and match compatriot Richard Gasquet's haul of three titles.
He also improved his head-to-head record against Pospisil to 6-0 en route to the ninth title of his career, although his task became easier after the world number 132 took a tumble and appeared be hampered by a leg injury.
Wimbledon
Monfils withdraws from Wimbledon despite return to training
23/06/2022 AT 16:49
Monfils became the eighth French champion in 10 editions of the ATP 250 event and the world number nine will head to Rotterdam to defend his title next week full of confidence.
It was a disappointing day for 29-year-old Pospisil, who was looking to capture a first ATP title in his second final, having returned to form following back surgery in January 2019.
Roland-Garros
Ranking shake-up: Medvedev set for No. 1, Djokovic to fall, Nadal to No. 4
06/06/2022 AT 08:28
Roland-Garros
'We are getting old' - Federer, Nadal, Djokovic pay tribute to retiring Tsonga
24/05/2022 AT 18:22