Financial terms were not disclosed for the offer that was signed Friday, but ESPN reported that Spikes received a six-year offer worth about $32 million.
Spikes desperately wanted to escape the Bengals and asked new Bengals Coach Marvin Lewis not to exercise their right to match the offer to the transition player. The Bengals will not receive any compensation for losing their defensive captain.
"Takeo has been a good player for the Bengals and in the community, and I wish him well with his career in Buffalo," Lewis said. "Today is the beginning of a new day in the Bengal organization and in the shaping of the football team under my direction."
On Friday, the Bengals prepared for the departure of Spikes by signing free agent Kevin Hardy, who has played for Jacksonville and Dallas.
Spikes, the team's leading tackler four times in his five seasons with the Bengals, has made it clear he has had enough of the losing in Cincinnati. In his five years there, the Bengals were 19-61.
The Bills cleared more than $5 million in salary cap space earlier Friday when they traded receiver Peerless Price to the Atlanta Falcons for a first-round pick in the 2003 draft.
Buffalo General Manager Tom Donahoe signed free agent Jeff Posey last week to start at strongside linebacker, and likely will have Spikes on the weakside to surround middle linebacker London Fletcher.
The decision was part of a busy day for the Bengals, who also signed veteran cornerback Tory James to a four-year contract at undisclosed terms.
James, 29, has spent the last three seasons with the Oakland Raiders after four years with the Denver Broncos. He has 19 career interceptions, including four last season for the AFC champions.
"Tory is an excellent cover corner," Lewis said. "He has excellent size and speed, and like some of the other players we've brought in, he has experience in the postseason. He knows what it will take to get there again."
James started all three postseason games for the Raiders last season, and had a key interception in a divisional playoff win over the New York Jets.
Buffalo ranked 27th in points allowed last season, 29th against the run, and last in the league in takeaways.