Smith, 30, returns for a third season as Russell Wilson's backup. He has appeared in one game for Seattle -- a blowout win last season over the New York Jets in which Smith completed 4 of 5 passes for 33 yards against the team that drafted him in the second round in 2013.
"Blessed with another opportunity to showcase all the hard work but it's still just the beginning for me!" Smith posted on Twitter.
Smith spent his first four seasons with the Jets, starting 29 games between 2013 and '14, but didn't stick. He spent the 2017 season with the New York Giants and the next season with the Chargers.
For his career, Smith has completed 57.8% of his attempts with 29 touchdown passes and 36 interceptions.