Terms were not disclosed, but ESPN reported that Adams, 29, agreed to a four-year deal.
The 6-3, 330-pounder will enter his 10th NFL season in 2003. He spent 2002 with the AFC champion Oakland Raiders, six seasons with the Seattle Seahawks, and two with the Baltimore Ravens.
In 2000, Adams, who has 29 career sacks, started all 16 games for the Super Bowl champion Raiders.
"This is a first class organization, facility, administration, and staff," Adams said after his visit to Buffalo on March 8. "This organization is geared towards winning a world championship. As a player, a free agent especially, this is a situation that you want to be involved. From the janitor to the owner, they are committed to winning a championship, and that's something that every player wants to be a part of."
The Bills already have brought in linebackers Takeo Spikes and Jeff Posey via free agency this offseason. Monday's deal gives the Bills a huge defensive tackle to play alongside Pat Williams, giving Buffalo one of the NFL's better tandems at the position.