Mallace, 27, had spent his entire professional career, six seasons, with Montreal after being selected in the 2012 MLS SuperDraft, making 80 MLS regular season appearances and six MLS Cup Playoff appearances for the Impact. He's also won two Canadian Championships with Montreal, in 2013 and 2014.
"We are pleased to have secured Calum's services, as he's a prime-age player with veteran experience in our league," said Sounders FC general manager & president of soccer Garth Lagerwey in a statement. "Like the addition of Lamar Neagle earlier this week, Calum gives our coaching staff another experienced option for the most critical portion of our season. We're excited to welcome him to Seattle."
Mallace is a native of Scotland who played collegiate soccer at Marquette.