Westerman, a native of Brampton, Ont., had 17 tackles and three sacks last season over 16 regular season appearances, eight of which were as a starter.
Westerman was selected in the first round (third overall) by B.C. in the 2012 CFL draft out of Eastern Michigan.
The Lions also agreed to extensions with receiver Kito Poblah and long-snapper Mike Benson.
Poblah, form Montreal, had 15 receptions for 182 yards and a touchdown during his season with the Lions after coming to Vancouver in an
Benson, from Winnipeg, joined the Lions midway through the 2014 campaign when injuries sidelined veteran long-snapper Jordan Matechuk. The former Edmonton Eskimo and Toronto Argonaut finished the year with four special teams tackles.