Alonso, voted Seattle's most valuable player in 2010, enters his third season with the team.
"We are thrilled to extend Ozzie's contract for an additional two years," said general manager Adrian Hanauer. "Through his hard work, intensity and heart, Ozzie is certainly deserving of the reward of a bigger and longer contract."
"Ozzie has been a vital part of the Sounders' success," said head coach Sigi Schmid. "His work rate, defensive tenacity and passing are important every game, and I am happy he will be with us for a while."
Alonso, 25, only scored one goal last season, but has made a name for himself thanks to his defensive efforts. Nine of the Sounders 11 shut-outs came in games that Alonso participated in.
"I am very pleased with the attention I have been given by the club, and I appreciate the confidence that Adrian and Sigi have shown in me," said Alonso. "I am very happy with the team and the fans here in Seattle. The fans have treated me well and I will give them and the club my best every time I step on the field, and I will work hard to bring success to the Sounders."
The former USL Rookie of the Year with Charleston, has been capped 17 times by the Cuban National team.
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