The team announced the signing Friday morning.
The 26-year-old Laurier Station, Que., native will reportedly earn US$3.5 milion per season.
Desharnais was third in team scoring last season with career highs of 16 goals and 60 points.
He has eight goals and eight assists in 26 games this season.
The five-foot-seven playmaking centre was signed as an undrafted free agent by Montreal in 2008.