ESPN first reported Williams' scheduled visit with Buffalo earlier on Tuesday.
The Associated Press learned of the visit from a person who spoke on the condition of anonymity because talks are ongoing.
Williams was the top pick in the 2006 draft by the Houston Texans, who elected against re-signing him this offseason. A two-time Pro Bowl starter, Williams has 53 sacks in six seasons, but was limited to playing just five games last season as a result of a torn chest muscle.
The Bills, coming off a 6-10 finish, pegged improving their pass rush as one of their top offseason priorities.