Gradkowski's agent, Rick Smith, confirmed the deal through his twitter account Wednesday.
The 30-year-old Gradkowski, a Pittsburgh native, played the past two seasons for the Cincinnati Bengals. He has thrown for 4,057 yards with 21 touchdowns and 24 interceptions in 36 career games.
The Steelers were 6-3 last season when Roethlisberger was sidelined by shoulder and rib injuries. Pittsburgh went 1-2 in his absence and continued to stumble upon his return, finishing a disappointing 8-8.
The Steelers also released guard Willie Colon. Pittsburgh earlier this week agreed to a three-year contract to re-sign guard Ramon Foster, who might get a chance to compete for Colon's vacated starting spot at left guard in 2013.
ESPN NFL Insider Adam Schefter and ESPN.com senior writer John Clayton contributed to this report.