The River Hawks (6-11, 2-2 American East) scored the first six points of overtime, including four from Harris. UMBC (5-14, 1-3) cut it to 91-89 with 18 seconds remaining but Logan Primerano and Harris both added a pair of free throws.
Harris made two free throws with 16 seconds left in regulation for an 83-80 lead. UMBC's Jairus Lyles missed a 3-pointer with four seconds left but Will Darley got the rebound and passed to Joe Sherburne who hit a last-second 3-pointer.
Harris was 8-of-13 shooting, including 5 of 9 on 3-pointers and 11 of 12 on free throws off the bench, which contributed 59 points. Jahad Thomas added 19 points and Tyler Livingston 15 plus nine rebounds.
Lyles scored 29 points with Rodney Elliott adding 24 plus seven rebounds for UMBC.