Nick Emery scored 27, Chase Fischer added 19 points and Kyle Davis had 11 with 14 rebounds for BYU (4-1). Emery and Fischer hit five 3-pointers apiece as the Cougars made 13 of 27 (48 percent) from behind the arc.
Belmont (4-3) led 49-47 early in the second half but BYU hit 3s on four consecutive possessions - the first two by Emery - during a 15-4 run that put the Cougars in front, 62-53, for good.
After a jumper by Mack Mercer cut Belmont's deficit to seven, Fischer hit back-to-back 3-pointers and the Bruins didn't challenge again. BYU led by as many as 18.
Craig Bradshaw scored 26 on 9-of-12 shooting, including 6-of-9 3-pointers, and Mercer had 16 points for Belmont.