The Steelers released Williams in March in one of their moves to get under the salary cap. He told Fowler then that he had a lot of football left and was eager to provide veteran leadership and play for a team that needs it.
He started 14 games last season, finishing with 70 tackles and three sacks. His usage jumped significantly last season after the departure of Mark Barron, playing 65% of defensive snaps, up from 37% the year before.
Williams, 31, was a full-time starter for the Steelers in three of the past four seasons.
A sixth-round draft pick by Pittsburgh in 2013, he has appeared in 121 games, with 69 starts, and has two interceptions (including a pick-six), 20.5 sacks and 333 solo tackles in eight seasons with the franchise.
Williams has indicated that he will think about retirement after this season and has considered getting into coaching.
ESPN's Brooke Pryor contributed to this report.