The 29-year old USA international heads to Washington with the Sounders receiving allocation money in exchange.
"Eddie was a valuable part of our team the last two seasons," said Sounders general manager Adrian Hanauer of the 61-times capped US attacker.
"Eddie is a tremendous player who gave us some wonderful moments. This was an opportunity for us to receive allocation money that can strengthen our club in many areas. We wish him the best in DC."
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