Tom Pelissero of NFL Network reported the news.
Martin, 30, rushed for 723 yards and four touchdowns during his lone season in Oakland. He was the odd man out on the depth chart after the Raiders drafted Josh Jacobs, with the team apparently choosing Jalen Richard as his backup.
A two-time Pro Bowler, Martin spent his first six NFL seasons with the Tampa Bay Buccaneers. He rushed for 1,402 yards and six touchdowns during the 2015 season and was rewarded with a five-year, $35.8 million contract but tested positive for Adderall in 2016, voiding the guaranteed money on the deal. The Bucs released Martin after poor performances in 2016 and 2017.
Martin took over for an injured Marshawn Lynch midway through last season in Oakland, running effectively but without much burst. The Raiders re-signed him in May after Isaiah Crowell suffered a torn Achilles during offseason workouts.
Martin said he was ready to embrace a role as a mentor to Jacobs in the spring.
"Yeah, it's not something I'm used to but it's something that I want to do," Martin told reporters. "I've been in the whole spectrum of good-to-bad in this league and if I can teach somebody, if I can teach Josh on what to do, what not to do, how to stay in on the narrow path and just keep his eye on the goal, then I'm here for that. And yeah, I'm grateful."
Martin has rushed for 5,356 yards and 30 touchdowns during his NFL career but has been wildly inconsistent. More than half of his career rushing yards were during his two Pro Bowl seasons, and he's rushed for less than 500 yards four times.