Jennings, who played 10 games for the Jaguars last season, initially tweeted unofficially that he had joined before deleting it from his account.
However now that the move is official, Jennings is looking forward to this new stage in his career.
"Its official! I'll be rocking that Black and Silver this yr," he said. "#Raidernation let's go make it happen!"
Jennings rushed for a total of 944 yards from 224 carries during his three seasons at Jacksonville.