A league source told ESPN NFL Insider Adam Schefter that the 49ers received a sixth-round pick in the 2011 NFL draft in return for Balmer.
Balmer left the 49ers last Monday for what coach Mike Singletary called personal reasons.
Balmer, the team's first-round draft pick in 2008 out of North Carolina, missed his fifth straight practice Friday before the team traveled to Indianapolis for its exhibition opener against the Colts.
Balmer's absences had been unexcused since Wednesday. Balmer met Wednesday night with Singletary, but still was a no-show at training camp.
"I moved on the last time we talked. I was done," Singletary said. "When we talked about it, when he wasn't here that morning, I was done. I was already on up the road. It's nothing I really look back at. I'm thankful he's with another team and hopefully he's happy. Whenever I have a player like that, I'll always miss that player in that capacity because I think he's a fine young man. But obviously there are some things he needed to express and deal with. I'm happy for him."
Singletary wasn't concerned about letting Balmer go to an NFC West opponent.
Former 49ers general manager Scot McCloughan, who abruptly departed the club in March, was hired by the Seahawks as a senior personnel executive in June. He is the right-hand man to general manager John Schneider.
Since McCloughan and the 49ers parted ways, San Francisco's operations have been run by Trent Baalke. The 49ers scored 34 straight points and rallied to a 37-17 win over the Colts on Sunday.
The Seahawks expected Balmer to arrive in town Monday night and be ready to practice Tuesday. Coach Pete Carroll said a roster move would be made once Balmer got there.
"He's a big strong kid," Carroll said. "He's another guy that can play defensive end and three-technique for us. He's got a great lower body. He's real physically capable of standing up against the big O-linemen we're going to play in our division. He was active. He's a good technician. We did a lot of homework on him to understand about his background and in college and had good information. What's happened is in the past as far as we're concerned and we feel real good about him getting in here and helping us right away."
Last Thursday, the 49ers signed rookie free agent defensive tackle Will Tukuafu to a two-year contract to help fill the void left by Balmer's absence. Tukuafu, out of Oregon, was originally signed by the Seahawks before they released him June 22.
The 23-year-old Balmer is entering the third year of an $8 million, five-year deal he signed as a rookie. He has yet to start a game and has recorded no sacks in his first two NFL seasons. He has 11 career solo tackles and has combined on eight others in 27 games.
Information The Associated Press contributed to this report.