Armstrong was originally signed by the Rams as an undrafted free agent in May 2013.
As a rookie, Armstrong appears in all 16 games though played primarily on special teams. He racked up three tackles last season on defense to go along with his 12 special teams tackles.
This year, the 6-foot-3, 234-pound linebacker played in four games.
In order to make room for Armstrong, the Raiders placed Kaluka Maiava on injured reserve.