Cooper, who was plucked off Major League Soccer waivers, is a veteran of eight MLS seasons. His 75 career regular-season goals rank him 18th all-time. He's also registered 15 assists, and started 172 of 210 games. He's appeared in seven MLS playoff matches.
"We have spoken about wanting to add quality depth to our attack and we have done so with this signing," Impact technical director Adam Braz said in a release. "Kenny has scored 75 goals in our league and that says a lot about his abilities. He is an excellent person who will fit in well with our group."
The Impact dropped to 0-2-2 on the season with Saturday night's 3-0 loss to the Houston Dynamo.
Cooper played 22 regular-season games last season, collecting three goals and four assists for the Seattle Sounders. He had two 18-goal MLS seasons, with FC Dallas in 2008 and the New York Red Bulls in 2012.
Cooper began his career with Manchester United, then was loaned to Oldham Athletic in League One. He also played in the German second division with 1860 Munich, and the English second division with Plymouth Argyle.
Cooper was scheduled to fly to Montreal on Monday and train with the team Tuesday.