With the decision before Monday's deadline for such a move, Spencer will be guaranteed a base salary of about $8.8 million next season.
Spencer has started 53 of 76 games in his five seasons with the Cowboys, starting all but one game the past three seasons. Spencer had 66 tackles with six sacks and four recovered fumbles last season.
Dallas has used the franchise tag only two other times, in 2002 on offensive lineman Flozell Adams and in 2008 on safety Ken Hamlin. Adams and Hamlin both later agreed to contract extensions.
Spencer has 21 1/2 career sacks.