The 33-year-old Nadal won the event for the third time in Montreal. The first came in 2005 at age 19 over Andre Agassi, and the second in 2013. Nadal won in Toronto in 2008 and 2018.
The second-ranked Spanish star won his 83rd singles title and third of the year, following clay victories in Rome and at the French Open. He extended his tour record for Masters 1000 titles to 35, two more than top-ranked Novak Djokovic.
Nadal had a walkover in the semifinals Saturday when Gael Monfils withdrew because of an ankle injury.
Medvedev, from Russia, was seeded eighth.