Seattle announced the signing on Wednesday. Smith, 28, completed 1-for-4 passing attempts for eight yards, while not throwing a touchdown or an interception in five games last season for the Los Angeles Chargers.
The second round pick in the 2013 NFL Draft played his first four seasons with the New York Jets, before signing with the New York Giants prior to the 2017 season. Smith has completed 57.7 percent of his passes for 6,182 yards, 29 touchdowns and 36 interceptions in 40 appearances during his six-year NFL tenure.
The West Virginia product hasn't started more than one game in a season since 2014.
Smith's signing means that the Seahawks have now signed three different quarterbacks this off-season. The Seahawks gave Wilson a four-year, $140 million pact in April. Seattle also signed former Denver Broncos backup Paxton Lynch to a one-year deal this off-season.
Seattle waived undrafted rookie quarterback Taryn Christion on Monday.