The Raiders later announced the move.
Leinart will back up starter Carson Palmer in Oakland and will be reunited with offensive coordinator Greg Knapp, who was his position coach the past two seasons in Houston.
Leinart becomes the Raiders' only backup quarterback to have thrown an NFL pass. Terrelle Pryor, who was taken last summer in the NFL supplemental draft, played one game last season and committed a penalty his only time on the field. Rhett Bomar spent two years on the New York Giants' practice squad.
Leinart reached the deal after showing the Raiders he has fully recovered from a broken left collarbone that cut short his 2011 season. The team also brought in Peyton Manning's former Indianapolis Colts backup, Jim Sorgi, for a tryout, but decided to go with Leinart.
Leinart took over for injured Texans quarterback Matt Schaub midway through last season, but suffered a fractured collarbone that ended his year in the first half of his first start Nov. 27 at Jacksonville. The Texans cut Leinart in March.
Leinart spent his first four years in the NFL with the Arizona Cardinals, who drafted him 10th overall in 2006, but he struggled. He started 11 games as a rookie but just seven games in the past five seasons.
Leinart broke his collarbone after five starts in 2007, then was beaten out for the starting job the next season by Kurt Warner.
Leinart spent two years behind Palmer at USC, including when Palmer won the Heisman Trophy in 2002. Leinart took over the Trojans the next season and won two national championships and the Heisman in 2004.
Follow Steve Wyche on Twitter @wyche89
The Associated Press contributed to this report.