Per Major League Soccer and club policy, terms were not disclosed.
Seamon, 22, was selected by Seattle with the 27th pick overall in the 2010 draft. After initially training with Sounders FC in the preseason, Seamon re-joined the team on May 18 following his graduation from Villanova University. He played as a reserve in the friendly with Argentine side Boca Juniors, scoring in the 70th minute on his first shot and first touch.
"We were excited when we had the opportunity to draft Michael," said Sounders FC head coach Sigi Schmid. "He's a player that can play number of different positions in midfield, and brings a special confidence and ability to our team."
Seamon is a two-time All-Big East selection, spending all four years at Villanova and leading his club in total points each year. He started all 18 games played as a senior, contributing five goals and seven assists, and was named first team All-Big East.