NFL Network's Adam Schefter reported Sunday that the deal included $10.5 million in guaranteed money.
Clayton, the 2004 NFL Offensive Rookie of the Year who has struggled since that strong debut, agreed to the contract two days after becoming an unrestricted free agent.
As a rookie, Clayton, a first-round draft pick from LSU (15th overall), had 80 catches for 1,193 yards and seven touchdowns -- the fifth-best rookie season in league history. But Clayton's production fell off after that, and he lost his starting job at times. He did start nine games last season after Joey Galloway was injured.
In the last four years, Clayton has been plagued by injuries and inconsistency, and his best season was 2008, when he had 38 receptions for 484 yards and one touchdown.
Clayton, who has scored just two touchdowns since his rookie season, has 205 career catches for 2,706 yards and nine touchdowns.
The deal means the Bucs, who are rebuilding under new head coach Raheem Morris, will return both of their starting wide receivers from last season. Last week, the team placed the franchise tag on Antonio Bryant, assuring him a one-year contract worth $9.88 million.
The Associated Press contributed to this report.