Major League Soccer announced the decision by an independent review panel on Wednesday. Seattle had submitted a request for review after Alonso was issued the red card in second half stoppage time of the Sounders 1-0 loss to San Jose on Saturday.
Alonso was issued the red card by referee Sorin Stoica after making contact with San Jose defender Dan Gargan. The review panel determined the contact did not reach the threshold of violent conduct.
It's the second time this season the Sounders have successfully had a red card rescinded. Forward Obafemi Martins was issued a red card in late May against Chivas USA that was later overturned on review.