The Colts announced the signing of Nix on Saturday. Financial details of the contract weren't disclosed.
The Steelers released Nix last month after five seasons with the team. The 28-year-old fullback played in only three games last season before ending the year on injured reserve.
Nix, a converted defensive lineman out of Kent State, entered the NFL as an undrafted free agent with the Atlanta Falcons in 2014. He joined the Steelers in 2015 and eventually earned his lone trip to the Pro Bowl in the 2017-18 campaign.
In 60 career games with the Steelers, Nix had four carries for four yards and one touchdown, and one reception for 12 yards. He was also a key contributor on special teams, and led the way for All-Pro running back Le'Veon Bell, who had 321 carries for 1,291 yards and nine touchdowns in his final season with the Steelers.
The Colts didn't have a fullback on their roster last season. Indianapolis will enter the 2020 campaign with Marlon Mack, Nyheim Hines and Jordan Wilkins in the backfield.