Leftwich, the source indicated to Clayton, will be scheduled to make $2 million this year. He will have a $3.5 million roster bonus in 2010 that has to be paid at the start of the free-agency period. If they pay it, Leftwich could make $5.5 million in 2010.
The Washington Redskins and Pittsburgh Steelers also were interested in Leftwich, who joins a depth chart in Tampa that includes Luke McCown, Josh Johnson, and Brian Griese.
Leftwich, 29, a first-round draft pick by the Jacksonville Jaguars out of Marshall in 2003, won a Super Bowl ring last season as a backup with the Steelers.
The 6-foot-5, 250-pounder has started just eight games since 2006, his last season with the Jaguars. He played for the Atlanta Falcons in 2007.
Leftwich passed for 303 yards with two touchdowns in five appearances with the Steelers last season as Ben Roethlisberger's backup.