Manuel, a former first-round pick, was a disappointment in four seasons with the Buffalo Bills.
The Bills believed Manuel could be their franchise quarterback when they selected him 16th overall in 2013, making the Florida State product the first quarterback chosen in what became a lackluster draft class at the position. No quarterback drafted in 2013 remained a full-time starter by 2016.
Manuel, who will turn 27 on Saturday, is expected to compete with second-year quarterback Connor Cook for the backup spot behind Raiders starter Derek Carr, who will be coming back from a broken right fibula suffered in Week 16.
Manuel's arrival likely means the end of Matt McGloin's time in Oakland after four seasons. McGloin started seven games for the Raiders, including last year's regular-season finale at Denver, and went 1-6.
Cook, a fourth-round pick last spring who had not dressed for a game until the season finale, started the Raiders' playoff loss at Houston.
Manuel started 10 games for Buffalo as a rookie, completing 58.8 percent of his passes for a 77.7 rating. He missed six games because of knee injuries.
Doug Marrone, Buffalo's head coach at the time, benched Manuel after he went 2-2 over his first four starts in 2014, replacing him with veteran Kyle Orton.
Rex Ryan turned back to Manuel for two games in 2015 after starter Tyrod Taylor was sidelined by injury, but Buffalo lost both games
Manuel did not start again until the 2016 regular-season finale, when he went 9-of-20 passing for 86 yards and a lost fumble before being replaced by rookie Cardale Jones for the fourth quarter in a 30-10 loss to the Jets.
ESPN's Mike Rodak and Paul Gutierrez contributed to this report.