Smith was initially loaned to the Sounders for the duration of a year, with a return to Bournemouth set on 31 July 2019. However, the Sounders and Bournemouth agreed to extend the loan until the end of the 2019 MLS season. Smith started each game during the team's run in the 2019 MLS Cup Playoffs, helping the Sounders reach their third MLS Cup Final in four years. On 10 November 2019, Smith started in the 2019 MLS Cup Final, which the Sounders would win. Smith became the first Australian footballer to lift the MLS Cup trophy.