The 25-year-old, in his first tour-level final in 16 months, won 7-5 6-1 in one hour and 21 minutes.
Edmund, seeded eighth at the event, converted his only break point in the first set to edge ahead, but demolished his opponent in the second, seizing control with shots that seemed to get more powerful as the match went on.
He earned an early break and took a 3-0 lead in the second set before Seppi left the court to receive treatment at 4-1. The Italian seemed to be slower on his return, making it even harder to chase down Edmund’s shots.
Seppi did save a match point, but Edmund – whose last success came in Antwerp in 2018 – was in fine form with his serve as 11 aces demonstrated his superiority, as well as the fact he did not face a single break point.
The Frenchman, appearing in his third final in five years in Rotterdam, made a flying start, breaking his opponent’s serve twice en route to serving out the opening set.
Monfils broke the 19-year-old’s serve again early in the second set and looked poised for a comfortable win until Auger-Aliassime saved four match points and broke back to make the score 5-3.
Though the Canadian was able to hold serve in the next game, Monfils served it out once more to take the win and title after just 87 minutes on court.
Meanwhile, Norwegian Casper Ruud clinched his first tour title by beating Pedro Sousa 6-1 6-4 in the Argentina Open final.
The 22-year-old did not face a break point in his nine service games and broke his Portuguese opponent three times on his way to a straightforward win.