Traore, 32, joins from French side Marseille, but is best known for his time at Liverpool, where he played between 1999 and 2006. The former Malian international was part of the Reds' Champions League winning side in 2005.
"Djimi has a great deal of experience and has played in the top leagues in Europe," said Sounders FC sporting director Chris Henderson. "He's a very good defender and makes smart decisions with the ball coming out the back. Djimi brings us confidence and leadership in the back."