The 28-year-old previously played with Seattle in 2009 and 2010, and most recently spent the 2015 season with the Houston Dynamo. Per league and club policy, terms of the agreement are not being disclosed.
"We're happy to have signed Nathan and he is eligible to play [in the CONCACAF Champions League] tonight against Club América," said Sounders general manager & president of soccer Garth Lagerwey in a statement provided by the team. "It's always an exciting addition when a former Sounder comes back and rejoins the team."
Sturgis started 15 of his 18 appearances for the Houston Dynamo in 2015, recording one goal and two assists in his 10th professional season in MLS. Originally drafted by the LA Galaxy 12th overall in the 2006 MLS SuperDraft, Sturgis spent time with the Galaxy and Real Salt Lake before being selected by Sounders FC in the 2008 Expansion Draft. Sturgis made 29 appearances over two seasons in Seattle, scoring his first career goal against Columbus on September 18, 2010. In 2013, while with the Colorado Rapids, the Clemson product set career highs in games played (25), starts (23), minutes (2,037) and goals (four).
"Nate is a solid player and an accomplished player who is technically very sound," said Sounders head coach Sigi Schmid. "He has two good feet, hits a great ball and is a very good passer. He is also a good guy in the locker room who gives us a bit of of a veteran presence coming off the bench in the midfield."