Details of the deal were not disclosed and it will not be finalized until after Bostic undergoes a physical on Monday.
The 26-year-old Bostic played 14 games for Indianapolis last season, getting 97 tackles and one sack. Bostic injured his right knee against Denver in December and finished the season on injured reserve. Pittsburgh is Bostic's fifth stop.
Bostic began his career with Chicago after being taken in the second round of the 2013 draft. He played two seasons in Chicago then one in New England before the Patriots traded him to Detroit in 2016. He missed the entire season in Detroit with a foot injury before joining the Colts.
The Steelers are in need of depth at inside linebacker with Ryan Shazier already ruled out for 2018 because of a spinal injury suffered in Cincinnati in December.
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