According to USA Today, Flynn was scheduled to workout in Buffalo before the Bills played the Bengals Sunday.
The former Packer, Seahawk and Raider worked out Monday morning in front of coach Doug Marrone, offensive coordinator Nathaniel Hackett and Bills General Manager Dough Whaley before inking what has been reported as a one-year deal.
While details of the contract are not known, the Raiders will pay Flynn $6.5 million this season.
Flynn was released by Oakland last week after losing his starting job to Tyrelle Pryor, similar to how he was beaten out by Russell Wilson in Seattle last year.
The former LSU Tiger was drafted by the Green Bay Packers in 2008 and spent four years backing up Aaron Rodgers.
The Bills named Thad Lewis their starting quarterback last week. Lewis was expected to remain the starter until he sprained his ankle in Sunday's 27-24 overtime loss to Cincinnati.
As of this report, undrafted rookie Jeff Tuel is the only other active quarterback on the roster.
It is not known if Flynn will play this week against Miami.