Rashawn Rembert's 3-pointer tied the game at 72-all for ETSU (8-4, 3-1 Southern Conference) with 2:19 left. Riley gave the Buccaneers the lead for good on a pair of free throws with 1:38 left, Lester Wilson made one to make it 75-73 and Riley made one with 21 seconds to cap the scoring.
Samford's Christen Cunningham missed a jumper at the buzzer.
The Bulldogs (7-10, 0-4) closed the first half with 12 straight points to lead 34-32 at halftime and neither team led by more than three in the second half.
Rembert had 18 points and Wilson had 17 for ETSU. Marcus Johnson was 5 of 6 from 3-point range and led Samford with 20 points.