The 27-year-old Thomas started a career-high 10 games for San Diego in 2013, finishing with 30 tackles and an interception. The 6-foot-3, 330-pound Thomas gives the Steelers some much-needed depth after backup nose tackle Al Woods and defensive lineman Ziggy Hood left via free agency earlier in the week.
Thomas will compete with incumbent Steve McLendon for playing time. Thomas was a 2010 fifth-round pick out of North Carolina who worked his way into a starting job for the Chargers.
Pittsburgh still needs help at defensive end, where Cam Heyward is the only lineman on the roster with extensive playing experience.
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