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.