Kashif Watson scored 15 points and Idaho handed No. 25 Portland its second straight loss since entering the national rankings for the first time in 50 years with a 68-48 victory Sunday night.
Luke Sikma scored 11 points to lead Portland (5-3), which also lost 86-82 to Portland State on Wednesday. The Pilots hadn't been ranked since February 1959.
It was Idaho's first win over a ranked team since it beat then-No. 16 Iowa in the first round of the 1982 NCAA tournament.
The Vandals (5-2) led the entire way, taking a 34-18 lead at the half. Idaho then went on a 14-2 run early in the second half to take control.
A pair of free throws by Steffan Johnson with 1:12 left gave Idaho its largest lead of the game - 67-42.
Idaho shot 52 percent from the field and 43 percent from behind the 3-point line, hitting 6 of 14 attempts.
Both teams turned the ball over 17 times but Idaho outscored Portland 23-8 on points off turnovers.