Walden worked out for the Seahawks earlier this month.
Walden's addition reinforces Seattle's pass rush, which might be the biggest question mark on Seattle's roster. Defensive end Dion Jordan, a projected starter opposite Frank Clark, is on the physically unable to perform list because of what coach Pete Carroll has described as a stress-related issue in his leg. Jordan's status for Week 1 remains uncertain.
"He's making progress. He's making progress but it's still a while, it's still probably three weeks before we make an assessment," Carroll said of Jordan. "It's probably that last week of preseason when we'll try to see if we can push for that and see how that goes. That's what we'd like to get done if we could."
Clark, meanwhile, had been limited in practice until this week as he worked his way back from hand surgery.
Walden, who turns 33 on Aug. 21, ended up being a successful late-July free-agent pickup last season for a Tennessee Titans team desperate for pass-rush depth. He produced four sacks as the Titans' top reserve outside linebacker and also stepped in for two spot starts when Derrick Morgan was injured.
Walden has stuck in the NFL, playing for five teams, due to his ability to get after the quarterback. He had 11 sacks in 2016 with the Indianapolis Colts and has 35 in 10 seasons for the Miami Dolphins, Kansas City Chiefs, Green Bay Packers, Colts and Titans.
ESPN's Brady Henderson and Cameron Wolfe contributed to this report.