The top-seeded Bruins (25-7), the defending tournament champions, used an 18-3 first-half run to take control after No. 8 seed Jacksonville held a 19-18 lead early. Belmont led 39-30 at halftime.
The Bruins, who won their 12th in a row, also got 18 points from Scott Saunders and 13 from Mick Hedgepeth.
Jacksonville (8-22) was led by Aloys Cabell, who had 17 points, while Glenn Powell had 12.
The Bruins shot 39.7 percent from the floor, hitting 23 of 58 shots, and they made 6 of 17 attempts from 3-point range. Belmont had a big advantage at the free throw line, hitting 24 of 31. The Dolphins had 13 attempts at the foul line, hitting 9.
The Bruins also had a 46-28 rebounding edge, including 15 offensive rebounds.