Cox, a two-time league all-star, received the award Thursday night at the CFL's awards banquet.
Defensive end Charleston Hughes of the Calgary Stampeders was the other finalist in voting by the Football Reporters of Canada and eight CFL head coaches.
Cox, 30, a native of Columbus, Ohio, was a key performer in a Montreal defence that allowed a CFL-low 314.3 yards per game. The five-foot-nine, 185-pound linebacker led the league in tackles (club-record 115), and had a team-high 12 sacks and four interceptions in his eighth season with the club.
The six-foot-one, 244-pound Hughes terrorized CFL quarterbacks, registering a league-high 18 sacks. The 29-year-old native of Saginaw, Mich., anchored a Calgary defence that led the league in sacks (63) and was second in fewest points allowed (22.9 points per game).