Motum hit 12 of 21 shots as the Cougars (15-15, 7-11 Pac-12) held a 30-10 scoring advantage in the paint and 36-25 rebounding edge in picking up just their second conference road win.
Maurice Jones scored 11 points to lead USC (6-25, 1-17), which closed the regular season with a ninth straight loss to sit at the bottom of the league standings.
The Trojans trailed by 17 points when they came alive late. Alexis Moore capped a 16-2 run with a corner 3-pointer to pull the Trojans within 41-38 with 1:57 remaining.
Washington State went without a basket for the final 9:09 of the contest.