The Cougars (5-3) outscored the Pilots 32-12 in the paint and had a 45-27 rebounding advantage.
Ryan Nicholas had 17 points and hit five 3-pointers for Portland (3-4).
The Pilots trailed 39-33 early in the second half, but Washington State started feeding the posts. Portland had no answer for the inside play of Motum, who scored 17 of his 29 points late in the second half.
Portland shot only 28 percent in the first half, connecting on only 7 of 25 shots.
Washington State shot 42.9 percent (24 of 56) for the game.
DouDou Gueye, a 6-foot-10 center from Senegal, has signed with Ball State as a graduate transfer from South Carolina State.
Duke freshman point guard Derryck Thornton Jr. is transferring to USC, his father announced on Friday to Scout.com.
We have changes at the top of the draft and a shakeup in the lottery, with teams starting to make their final decisions. Get the latest in Chad Ford's Mock Draft 9.0.