Davis, the Kansas City Chiefs' third-round draft pick in 2013 out of Arkansas, has played in 57 games in his four NFL seasons.
Davis, 25, spent the 2013-15 seasons with Kansas City before stints with the Green Bay Packers (two games) and Chiefs (nine games) in 2016.
"It's a wonderful situation," Davis said of signing with the Steelers. "You have a good running back, a good coaching staff, good offensive scheme and good special teams. I think it's a good place for me. I felt like with (coach) Mike Tomlin and what he is doing around here, the success that they have had, and they are looking good. I think I can help out."
Davis' career regular-season numbers include 250 carries for 805 rushing yards and 11 touchdowns, with 34 receptions for 271 yards and a TD in 57 games (two starts). He also returned 73 kickoffs for 1,960 yards (26.8 average) with two touchdowns.
In three career postseason games, Davis rushed for 109 yards and a touchdown on 28 carries, and caught nine passes for 46 yards and a TD. In addition, he made four kickoff returns for 196 yards (49.0 average) and a touchdown.
Davis said he is looking forward to working with star running back Le'Veon Bell. The Steelers used the franchise tag on Bell and hope to sign him to a long-term deal after he rushed for 1,268 yards and seven touchdowns in 12 games, and caught 75 passes for another 616 yards and two TDs during the regular season. Bell became the first player in NFL history to average 100 rushing yards and 50 receiving yards per game.
"I know Le'Veon. Great guy," Davis said. "Awesome football player. I think he has done a lot of good for the league. He has changed the way the position has been played with his footwork and patience. There is a lot I can learn from him, a lot that I have learned from him just by watching him.
"I think he is one of the best backs in the league. He is unique. He is different. His attention to detail, you can tell he watches film. It's going to be fun working with him."