Spencer is guaranteed to make about $8.8 million next season under the one-year deal announced Monday.
A first-round pick out of Purdue in 2007, Spencer has 21.5 sacks in 76 career games, including 53 starts. He has started 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 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.