The team officially announced Kelly's addition Wednesday.
Kelly, 32, played for the Oakland Raiders from 2004 to 2012, but was released by the team March 27. At 6-foot-6 and 325 pounds, he brings size and power to the interior of the Patriots' defensive line. He's been known for his durability, starting all 16 regular-season games in each of the past five seasons.
Kelly totaled 45 tackles and one sack in 2012. In the two seasons prior, he combined for 14½ sacks.
Kelly was praised by Patriots coach Bill Belichick before the 2005 opener against the Raiders.
"Tommy Kelly is one of the best defensive linemen in the league. He's an outstanding player," Belichick said at the time.
"Kelly plays everywhere. He's a little bit like Howie Long in that Howie Long was a guy that they would move along on the front. They would take Howie and mismatch him against whoever they thought the other team's worst lineman was. There is a little bit of that with Tommy Kelly too, find a guy that they want to go after and stick him there because he does have the flexibility to play outside and inside."
The Patriots have Vince Wilfork as their top defensive lineman. Fourth-year players Kyle Love and Brandon Deaderick split starting duties next to him in 2012. Both Love and Deaderick are entering the final year of their contracts.