The man, Alan Smith, had been initially considered a "person of interest" in the February killing of his estranged wife, Susann Smith. Bothell police later called him a suspect in the murder.
A police spokesman, Sgt. Ken Seuberlich, says detectives have been putting the case together and felt comfortable enough Thursday to make an arrest.
"Bits and piece of evidence continue to come in," Seuberlich said. "Detectives felt comfortable today to arrest him on the murder of Susann."
Smith was taken into custody at the home in Bothell where his wife was killed and was booked into the Snohomish County Jail in Everett.
In an interview with KOMO News last week Smith denied having any role in his wife's murder.
"She was the love of my life for a while and the mother of my children, and my heart breaks," Smith said.
Soon after the February murder, Smith moved back into the home where Susann was bludgeoned to death.
Then, on June 17, neighbors called police after they heard a woman screaming at the home. When officers arrived, they found Smith having sex with his girlfriend, Love Thai, in the front yard.
Smith was not arrested or cited for that incident, but police said he was a "person of interest" in his wife's murder and obtained a search warrant for the home to find Smith's passport.
On June 21, Smith was arrested for investigation of obstructing an officer, but was released from jail the next day after posting $10,000 bail and turning in his passport.
He was re-arrested Thursday and booked into jail for investigation of first-degree murder.
Police said Smith's girlfriend, Love Thai, is not a suspect in the homicide investigation.
Susann Smith was found dead of severe head injuries Feb. 12 in the bathtub. She worked for a mobile game developer in Seattle.
The Smiths had been involved in a divorce and a custody dispute over their two young children. The children are now in the care of Child Protective Services.