Ridnour already had declared for the NBA draft, but he could have withdrawn his name by June 19 and returned for his senior season by not signing with an agent.
Tuesday, he announced his plans to sign with Washington law firm Williams & Connolly, LLP, the same group that represents Tim Duncan, Grant Hill, Shane Battier, Andre Miller, and Chamique Holdsclaw, among others. Ridnour will be represented by Lon Babby and Jim Tanner.
"They represent some quality guys with good character who have had a lot of success in their careers, so I felt comfortable choosing them," Ridnour said. "It's time for me to do this."
He said he will use the next six weeks to work out for teams prior to the June 26 draft.
Ridnour said that with Coach Ernie Kent's help, he has gotten a good idea of where he might be selected.
This past season, he ranked second in the Pac-10 with 19.7 points per game and led the league with 6.6 assists per contest, earning conference Player of the Year honors. He then led the Ducks to the Pac-10 tournament title.
Ridnour, a native of Blaine, Wash., set school and Pac-10 records by making 62 consecutive free throws in January and February. He also established a school mark with 218 assists and tied another with 63 steals.
In school history, Ridnour is ninth with 1,399 points, third with 500 assists, second with 150 steals and third with 203 3-pointers.
He will become the second player in school history to leave early for the NBA, joining Terrell Brandon, who skipped his final year of eligibility in 1991.
"I prayed a lot, and it felt like the right time to make this decision," said Ridnour. "I think the Lord has a plan for me and this is part of it. With Coach (Ernie) Kent's help, I've been able to get an idea of where I might be chosen in the draft. You never know for sure, but I feel pretty good that I'll go somewhere in the first round."