The 34-year old ex-Grenada international midfielder heads to Seattle along with the No. 3 Allocation ranking, a 2014 second round selection in the 2014 SuperDraft and a second round pick from Chivas in the 2015 draft in exchange for Seattle's No. 15 Allocation ranking.
Sounders head coach Sigi Schmid was thrilled to welcome Joseph who will occupy a Designated Player spot on the roster.
"We're happy to add a player of Shalrie's experience to our team," said Schmid.
"He's been to MLS Cups before in his tenure with New England. He's a player that has proven himself to be a leader in game situations and a player of his quality can only help us be a better team as we move into this season. "