"Marshawn's ecstatic and looking forward to helping the Bills," agent Doug Hendrickson said. "All along, Marshawn told us he wanted to be in camp on time and instructed us to get the deal done."
Drafted 12th overall out of California, Lynch will earn $10.285 million in guaranteed money including bonuses, with the entire contract worth $18.935 million.
Buffalo drafted Lynch to replace former starter Willis McGahee, who was traded to Baltimore in March.
Lynch was the second running back selected in the draft, behind Oklahoma's Adrian Peterson, who was taken at No. 7 by Minnesota. Lynch finished with 3,230 yards rushing and 29 touchdowns, while adding 600 yards receiving and six TDs in 35 games spread over three seasons at California.
The Bills particularly like Lynch's versatility as a receiver, an added dimension that fits offensive coordinator Steve Fairchild's scheme and something with which McGahee struggled.
Lynch, who played quarterback in high school, even showed off a strong arm in college, completing three passes for 55 yards and two touchdowns.