The 27-year-old Armstrong tallied 15 goals and 14 assists for Atlanta last year. Aside from his potential to find the net, Armstrong is an aggressive hitting forward who brings toughness to Toronto.
The Canadian-born player from Saskatchewan had a breakout year, scoring a career best 22 goals, with the Thrashers in 2008-09. He also matched a career high that year with 40 points -- he had the same amount with Pittsburgh in 2005-06.
Armstrong and Leafs' captain Dion Phaneuf played together for the Red Deer Rebels of the WHL during the 2001-02 season.