Lee Soo-il, 62, and Lee Keum-yim, 60, were found dead on Christmas Day in their home in this community south of Tacoma.
Dispatchers received a 911 call at 5:30 p.m. Sunday from a person who had found the bodies, sheriff's spokesman Ed Troyer said.
The son was arrested Monday at a house near Rainier, Troyer said, adding officers also seized the vehicle the man was driving.
Korean translators were called in to assist, Troyer said.
No autopsies had been performed as of late Monday, said Jim Eggleston in the Pierce County medical examiner's office.
The only apparent witness was an adult daughter who wasn't injured.