The 29-year-old Johnson spent the last four seasons in Cincinnati, where he played 58 games -- including 12 starts -- recording a career-high 79 tackles in 2008. Johnson was originally drafted by the Arizona Cardinals in the fifth round of the 2006 draft.
Johnson gives the Steelers needed veteran depth at linebacker. Pittsburgh's other three top reserves -- Stephenson Sylvester, Jason Worilds and rookie Sean Spence -- have started a combined eight NFL games.
The Steelers also signed journeyman long snapper Matt Katula, who split time with the New England Patriots and Minnesota Vikings last season. To make room for Johnson and Katula, the Steelers released tight end Wes Lyons and placed receiver Connor Dixon on the injured/waived list.