Portland adds a well-known name to help shore up its left back position.
Silvestre, 35, is best known for the 10 seasons he spent with Manchester United between 1999 and 2008, where he won five Premier League titles and one Champions League. He also played two seasons with Arsenal between 2008 and 2010.
"Mikael will bring a tremendous amount of experience and a presence on the field and in the locker room,” said Timbers head coach Caleb Porter. “He is a quality player on both sides of the ball and his impact will certainly go a long way in helping to solidify our back line."
Silvestre, who was signed following a two-week trial, has 40 caps with the France national team and played in the 2006 World Cup. Most recently, he had been with Werder Bremen in the German Bundesliga.