"I'm very excited to be back into coaching, and it's a wonderful opportunity," Kent told ESPN.com.
Kent signed a five-year rollover deal, the school announced. It did not provide financial terms.
A news conference is set for Wednesday.
Kent, 59, coached the Ducks from 1997 to 2010 and went to the NCAA tournament five times, including Elite Eight appearances in 2002 and 2009.
Washington State athletic director Bill Moos was the AD at Oregon while Kent was the coach.
"I am excited that Ernie will be the leader of our men's basketball program," Moos said. "I have witnessed firsthand his many talents. He has proven that he can win championships in our conference. Together we saw tremendous success at his alma mater and I have every reason to expect to see the same at mine."
Kent was fired in 2010 after two straight seasons in Eugene without a postseason appearance. The Ducks were 8-23 in 2009 and 16-16 in 2010. He worked as an analyst for the Pac-12 network this past year.
"I'm looking forward to providing the kind of passion and leadership to help potential student-athletes understand how special WSU is," Kent said in a statement released by the school. "Getting back into coaching for me has meant finding the right program that matches up with my passion, my vision, my beliefs and my commitment; and I feel Washington State University fits all that criteria for me."
Kent has gone 325-254 (.561) as a head coach, including his tenure at Oregon and a six-year stretch at Saint Mary's.
Washington State talked to Boise State's Leon Rice but Rice decided to stay at Boise State.
ESPN.com's Andy Katz contributed to this report.