With the game tied at 82-82, Volkman buried a 3-pointer with 3:30 remaining to start an 11-4 run and give Washington State the lead for good. The Wildcats pulled within 87-84, but Nyseth put home a lay-up with 1:34 to go to give the Cougars an 89-84 advantage.
The Cougars shot 42 percent (32-of-75) from the field and 63 percent (21-of-33) from the free-throw line.
LaKeisha Taylor scored 23 points to lead five players in double figures for Arizona (15-7, 5-5), which has lost five of its last six games.
The Wildacts shot 52 percent (31-of-59) from the field and hit 20 of their 27 free throws.