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.
Mike Krzyzewski says he has good reason to be excited for next season thanks to a combination of Duke's incoming freshmen and a core of experienced upperclassmen.
Georgia guard Juwan Parker has been awarded a medical redshirt, leaving him with two years of eligibility, after missing the 2015-16 season with a partially torn Achilles tendon.
Forward Trevon Bluiett has withdrawn from consideration from the NBA draft and will return to Xavier for his junior season.