The Citadel will face Wofford (17-12) in a semifinal game on Saturday.
Quinton Marshall's dunk, with 11:44 left in the first half, tied the game at 13 and sparked a 11-2 run by The Citadel (7-25), giving the Bulldogs a 22-15 lead. They wouldn't trail again.
The Citadel shot 52 percent from the floor (26 of 50) and 52.6 percent from 3-point range (10 of 19) while getting 31 points from its bench. Reserve P.J. Horgan scored eight points and grabbed career-high 12 rebounds as the Bulldogs out-rebounded the Spartans 36-29.