The deal could be worth $2 million if he meets his playing time incentives, a source told ESPN.com's John Clayton.
The team announced the deal earlier in the day for the starter with the Houston Texans from 2002 until last season.
Seattle's first preseason practice is Saturday.
Pitts started 114 games for Houston, first as a left tackle and then in 2006 as a left guard. A second-round pick for the Texans in 2002, Pitts is rejoining former Houston line coach Alex Gibbs in Seattle.
The Seahawks released Henderson and linebacker Anthony Heygood on Thursday.
Information from The Associated Press was used in this report.