Ellis made eight of 12 overall and went four for four from behind the arc in the second half.
USC (13-17, 7-12 Pac-12) has won back-to-back games — tied for its longest win streak of the season — for the fourth time. Kobe Johnson added 15 points and DJ Rodman scored 11 for the Trojans.
Jose Perez scored 19 of his 25 points in the second half for Arizona State (14-16, 8-11). Adam Miller scored 18 points and Frankie Collins added 11, six assists and four steals.
Shawn Phillips Jr. scored five consecutive points, capped by his dunk that gave Arizona State a 51-49 lead with 11:45 to play, but Ellis hit a trio of three-pointers — and Oziyah Sellers made another — in a 12-2 run that made it 61-53 with 8:54 left and the Trojans led the rest of the way.
The Sun Devils shot 50% (24 of 48) from the field and 22 of 30 from the free-throw line but made just three of 18 from behind the arc. USC made 22 of 48 overall, 25-of-30 free throws and 12 of 24 from three-point range.
USC wraps up its regular season when it plays host to No. 5 Arizona on Saturday. Arizona State plays its finale Saturday when the Sun Devils visit UCLA.
