Dexter Kernich-Drew had a career-high 15 points and five assists. D.J. Shelton added 14 points including a 3-pointer down the stretch to help Washington State (4-3) with the win.
Idaho recovered from a 13 point deficit in the first half.
It would cut the lead to two points, 53-51, with 3:30 remaining in the game, but later, Shelton knocked down a 3-pointer as the shot clock expired to give the Cougars the spark they needed, scoring nine of the last 11 points for the Cougars.
Stephen Madison had 13 points for Idaho (1-4). The Vandals committed 11 turnovers in the contest, while Washington State had four.
The Cougars outscored Idaho 44-24 in the paint and shot 45.6 percent from the field.