"(Baltimore) was a team that was very aggressive in coming after Josh," Seattle general manager John Schneider said. "They were in a situation where they have a strong need. It has more to do with how Walter has stepped up and how (Marcus Trufant) is playing and some of the younger guys."
The move came just as the Seahawks were wrapping up practice, in which Wilson participated before receiving word of the trade.
Wilson, a second-round pick out of Maryland in 2007, has been with the Seahawks three seasons and started 23 of 28 games the last two seasons. He returned both of his 2009 interceptions for touchdowns.
"Josh is someone who has started a lot of games for a young player, plus he has been a good special-teams player, both as a kick returner and in coverages," Ravens executive vice president/general manager Ozzie Newsome said in a statement released by the team. "He adds depth to our secondary and will help us. ... We expect Josh to be taking snaps against the Jets on opening night."
Wilson averaged 27 and 25 yards per kickoff return his first two years in the league. He averaged 19.3 yards last season.
Wilson's status as an unrestricted free agent after this season also played a role in the deal. In Baltimore, he'll fill a need created when cornerback Domonique Foxworth tore up his knee during a pre-camp drill.
"With Josh, in terms of the value, it was at a level with him being an unrestricted free agent after the season," Schneider said. "It was at a point where we felt was definitely fair."
Carroll said Jennings will step into the starting role opposite Trufant. Jennings, nicknamed "Slim" because of his slender, 180-pound frame, has started 29 of 64 games since he was taken in the first round of the 2006 draft. His lack of size has led to Jennings getting isolated against bigger receivers, and he has just one interception in four seasons.
Thurmond will step into the fifth defensive-back spot less than a year after a devastating knee injury that cost him most of his senior season at the University of Oregon. He'll share some of his nickel duties with safety Jordan Babineaux and Roy Lewis. Running back Leon Washington will become the Seahawks primary kick returner.
"Walter has had a great camp. He's played beautifully," Carroll said. "He's played almost more than anybody else so we've gotten a great look at him and really excited about his contribution."
The Associated Press contributed to this report.