McKelvin will be part of a defense that's going through a transition under new coach Doug Marrone, who takes over after Chan Gailey was fired in January following three losing seasons. Marrone has brought in an entirely new coaching staff, which includes former New York Jets defensive coordinator Mike Pettine in the same role in Buffalo.
The Bills have also cut veteran safety George Wilson and linebacker Nick Barnett this offseason, while defensive end Chris Kelsay and linebacker Shawne Merriman have announced their retirement.
General manager Buddy Nix had expressed interest in re-signing McKelvin. At the NFL combine last month, Nix listed McKelvin along with safety Jairus Byrd and guard Andy Levitre as the top three pending free agents he would like to keep.
The Bills placed the franchise tag on Byrd to retain his rights last week.
Levitre, a four-year starter, remains unsigned and is expected to test free agency.