The team says Alonso, 28, has been a steady presence in Seattle's midfield since signing with the club in January, 2009. The three-time MLS-All Star was named Sounders FC MVP for four consecutive seasons.
"Ozzie is the ultimate competitor who has given everything to Sounders FC," Sounders owner and general manager Adrian Hanauer said in a statement announcing the signing. "His passion for the game of soccer and his drive to be the best are validated by being named team MVP four-consecutive seasons by his teammates. Ozzie is a key building block for our club and we are happy to sign him to a long-term contract extension."
Terms of the deal were not disclosed.