Joshua W. Nantz, 23, of Pullman, was arrested for investigation of first-degree assault Wednesday night after he came to the police station, Police Chief Gary Jenkins said in a statement.
Nantz was expected to make a court appearance Thursday.
Madeline A. Fouts, 21, was arrested earlier in the day for investigation of rendering criminal assistance and providing false statements to a public servant. Fouts was not directly involved in the assault on David Warner, Jenkins said. She was released from custody Wednesday evening.
Both arrests came after an anonymous tip was received last weekend, Jenkins said.
Warner remains in critical condition at a Spokane hospital. Police say he was beaten about 2 a.m. on March 30 while intervening in a fight between an acquaintance and a group of college-age people.
On Tuesday, WSU President Elson Floyd donated $10,000 to the reward fund to help catch the assailants.
"I strongly encourage anyone who knows anything about what happened that night or recognizes any of the individuals in the videos that have been publicly distributed to come forward," Floyd said in a statement.
Pullman Police Lt. Chris Tennant said earlier that witnesses had been slow to provide information. Surveillance video showed up to 15 people in the area of the fight, but only about half have come forward, Tennant said.
The incident occurred outside a bar located near campus, he said.
Walker was walking with an acquaintance who exchanged words with a group of young people. Warner stepped between them and was struck, police have said.
Warner, 41, is Native American and teaches in the WSU Department of Critical Culture, Gender, and Race Studies. Police have said there's no indication race was a factor in the beating.
But Floyd on Tuesday announced that he was creating a Commission on Campus Climate to address what he called "an underlying fear and anger among some on campus regarding issues of race and marginalization."
University spokesman Rob Strenge said Floyd does not necessarily believe race was a factor in the attack, but he was responding to members of the community who are speculating that race might be involved.
"Since the night of the attack there has been a great deal of concern among some students that it might be a hate crime," Strenge said. "His intention is to address those concerns."