In 2008, Alonso was signed by Seattle Sounders FC, for whom he quickly became a regular starter at defensive midfield.
In the 2009 MLS Season, he started 28 of 29 appearances in the regular season and playoffs.
He played the full 90 minutes in 26 games and all 210 minutes in both playoff games against Houston.
After missing three matches with a quadriceps strain, Alonso returned to score on a long-range blast against D.C. United on June 17.
He finished with one goal and three assists for the season.
All three assists came during first two matches.
Alonso also appeared in two U.S. Open Cup games, including 90 minutes in the final.

In 2010, Alonso returned from injury to help Seattle post the league's best record in the second half of the season.
He started 23 of 25 appearances in the regular season and playoffs, matching his 2009 totals with one goal and three assists.
He scored the game-winner against Chivas on October 15.
Alonso missed eight games due to a quadriceps strain that happened on May 8 and returned against FC Dallas on July 11.
Seattle was 11-3-5 in regular season matches when he played at least 45 minutes.
Alonso also appeared in two U.S. Open Cup and five CONCACAF Champions League matches.

In February 2011, Alonso signed a contract extension with Seattle through the 2014 season.
In 2011, he led field players with 33 appearances and 2891 minutes (ranked 12th among field players in MLS).
He played the full 90 minutes in each of his 32 MLS starts.
Alonso scored a career-high three goals and matched his high with three assists.
He scored the game-winner at Portland on July 10 by converting his first career penalty.
He finished second on the team with 54 shots.
He was selected as an MLS All-Star for the first time.
He scored both goals in 2–2 draw at Comunicaciones on September 27 in a CCL Group Stage match.
Alonso was named U.S. Open Cup Player of the Round for his performance in the final. (
He added late goal in a2-0 win over Chicago.)
He made 43 appearances and 40 starts across all competitions.

In 2012, Alonso was named to the MLS Best XI for the first time in after a stellar season in midfield, leading the team with 30 starts.
He compiled a team-high 2,688 minutes in regular season play, the second-highest total by a field player in franchise history behind his 2011 total.
He was named an MLS All-Star for the second straight season and voted team MVP for the third straight year.
 Alonso played 56 minutes in a 3–2 win over Chelsea in the 2012 MLS All-Star Game.
His lone goal of the season came on a 65th minute game-winner at Colorado on July 28 when he volleyed a 12-yard shot into the back of the net.
He was named the 2012 U.S. Open Cup Player of the Tournament after scoring four goals and two assists in four games, including the winner against Atlanta on May 30 and a 30-yard volley against Cal FC on June 5.
For the season he totaled five goals and five assists while leading the team with 42 starts and 3,749 minutes across all competitions.

In 2013, Alonso was named an inactive MLS All-Star, earning his third All-Star selection in his five-year MLS career.

After missing games in both the 2014 and 2015 MLS Cup Playoffs, Alonso was being actively shopped around the league by the Seattle Sounders front office due to durability concerns before the start of the 2016 MLS season.
A trade did not materialize, and Alonso went on to make 38 league and playoff appearances for the 2016 MLS Cup winning Sounders.
He picked up an injury during the playoffs for the third season in a row during the second leg of the Western Conference Finals against the Colorado Rapids when a collision led to a sprained knee.
Alonso played through the injury, playing all 120 minutes of the 2016 MLS Cup Final in a penalty shootout win over Toronto FC.
He required four pain killing injections before the Final and another four injections at half time to make it through the game.
Head coach Brian Schmetzer praised Alonso for his determination and service to the club after the victory.

Alonso passed 20,000 minutes played for Seattle on April 14,
In 2008, Alonso was signed by Seattle Sounders FC, for whom he quickly became a regular starter at defensive midfield. In the 2009 MLS Season, he started 28 of 29 appearances in the regular season and playoffs. He played the full 90 minutes in 26 games and all 210 minutes in both playoff games against Houston. After missing three matches with a quadriceps strain, Alonso returned to score on a long-range blast against D.C. United on June 17. He finished with one goal and three assists for the season. All three assists came during first two matches. Alonso also appeared in two U.S. Open Cup games, including 90 minutes in the final.
In 2010, Alonso returned from injury to help Seattle post the league's best record in the second half of the season. He started 23 of 25 appearances in the regular season and playoffs, matching his 2009 totals with one goal and three assists. He scored the game-winner against Chivas on October 15. Alonso missed eight games due to a quadriceps strain that happened on May 8 and returned against FC Dallas on July 11. Seattle was 11-3-5 in regular season matches when he played at least 45 minutes. Alonso also appeared in two U.S. Open Cup and five CONCACAF Champions League matches.
In February 2011, Alonso signed a contract extension with Seattle through the 2014 season. In 2011, he led field players with 33 appearances and 2891 minutes (ranked 12th among field players in MLS). He played the full 90 minutes in each of his 32 MLS starts. Alonso scored a career-high three goals and matched his high with three assists. He scored the game-winner at Portland on July 10 by converting his first career penalty. He finished second on the team with 54 shots. He was selected as an MLS All-Star for the first time. He scored both goals in 2–2 draw at Comunicaciones on September 27 in a CCL Group Stage match. Alonso was named U.S. Open Cup Player of the Round for his performance in the final. (He added late goal in a2-0 win over Chicago.) He made 43 appearances and 40 starts across all competitions.
In 2012, Alonso was named to the MLS Best XI for the first time in after a stellar season in midfield, leading the team with 30 starts. He compiled a team-high 2,688 minutes in regular season play, the second-highest total by a field player in franchise history behind his 2011 total. He was named an MLS All-Star for the second straight season and voted team MVP for the third straight year.  Alonso played 56 minutes in a 3–2 win over Chelsea in the 2012 MLS All-Star Game. His lone goal of the season came on a 65th minute game-winner at Colorado on July 28 when he volleyed a 12-yard shot into the back of the net. He was named the 2012 U.S. Open Cup Player of the Tournament after scoring four goals and two assists in four games, including the winner against Atlanta on May 30 and a 30-yard volley against Cal FC on June 5. For the season he totaled five goals and five assists while leading the team with 42 starts and 3,749 minutes across all competitions.
In 2013, Alonso was named an inactive MLS All-Star, earning his third All-Star selection in his five-year MLS career.
After missing games in both the 2014 and 2015 MLS Cup Playoffs, Alonso was being actively shopped around the league by the Seattle Sounders front office due to durability concerns before the start of the 2016 MLS season. A trade did not materialize, and Alonso went on to make 38 league and playoff appearances for the 2016 MLS Cup winning Sounders. He picked up an injury during the playoffs for the third season in a row during the second leg of the Western Conference Finals against the Colorado Rapids when a collision led to a sprained knee. Alonso played through the injury, playing all 120 minutes of the 2016 MLS Cup Final in a penalty shootout win over Toronto FC. He required four pain killing injections before the Final and another four injections at half time to make it through the game. Head coach Brian Schmetzer praised Alonso for his determination and service to the club after the victory.
Alonso passed 20,000 minutes played for Seattle on April 14, 
Osvaldo Alonso