On April 26, 2012, the Tampa Bay Buccaneers selected Martin with the 31st overall pick of the 2012 NFL Draft. On June 4, 2012, Martin signed a 5-year contract with the team. After a successful 2012 preseason with the Buccaneers, where he rushed for 97 yards on 27 carries to go with 2 rushing touchdowns, Martin was officially named as the starting running back entering the 2012 regular season by coach Greg Schiano.
On October 25, 2012, Martin rushed for 135 yards and 1 TD and caught 3 passes for 79 yards and 1 TD in a 36-17 win against the Minnesota Vikings. On November 4, 2012, Martin rushed for 251 yards and tied the NFL record for rushing touchdowns in a half with 4 in a 42-32 road win against the Oakland Raiders. In the game, Martin became the first player in NFL history to score touchdown runs of 70-plus, 45-plus, and 65-plus yards in a game, and is the second player in league history (along with Mike Anderson) to run for 250+ yards and four touchdowns in a game (coincidentally, they both ran for exactly 251 yards and were both in their rookie seasons). Martin also broke the franchise records for rushing yardage and touchdowns in a game. 
At the end of the 2012 NFL regular season, Martin finished with over 1450 yards rushing and nearly 500 yards receiving. Martin's 1926 yards from scrimmage in a single season ranks him second in Buccaneers' history behind James Wilder (2229 yards from scrimmage). His 1454 rushing yards broke the Buccaneers' single-season rookie rushing record that was previously held by Cadillac Williams (1178 rushing yards). On January 4th, 2013, Martin was announced as one of five finalists for the 2012 NFL rookie of the year award while earning a trip to the 2013 Pro Bowl as first alternate.