The 29-year old arrives at CenturyLink Field in exchange for allocation money and a third-round pick in the 2015 MLS SuperDraft.
Sounders head coach Sigi Schmid was thrilled to welcome the 11-times capped USA international to the club.
"Chad is a quality defender who brings experience to the Sounders," said Schmid in a press release.
"He is dominant in the air, has won an MLS Cup and been a league defender of the year. I look forward to working with Chad again and await his positive contribution to our team."
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