The move comes a day after the team signed wide receiver Ryan Grant.
Nelson retired from the NFL last week.
Raiders quarterback Derek Carr thanked his now former teammate on Instagram.
View this post on Instagram
@sethtroberts I appreciate everything you did for our team. From game winning touchdown after game winning touchdown. Your relentless work ethic that took you from an Undrafted rookie to someone who made a team, earned a starting spot, and always showed up when I needed you. From catching passes during the defensives set of plays every single day, to you even trying to run me over on a reverse haha! Thank you!
Roberts, 28, was scheduled to be paid a base salary of $4.45 million in 2019. The Raiders saved $4.65 million on their salary cap by releasing him.
Roberts had 45 receptions for 494 yards and two touchdowns last season, his fifth with the Raiders.
He has 158 receptions for 1,826 yards and 13 touchdowns in 62 games, including 25 starts.