The seventh-round draft choice out of Alabama is considered the top competitor to unseat Earnest Graham as the Bucs' No. 3 running back.
The former Crimson Tide back had back-to-back 1,000-yard seasons in 2004-05, rushing for more than 2,300 yards and scoring 11 touchdowns during that time. He earned all-Southeastern Conference first-team honors in 2005.
His numbers took a dip last season to 835 rushing yards, but he still averaged 4.0 yards per carry and caught 23 passes for 130 yard and a touchdown. In doing so he earned his second all-SEC second team selection (also in 2004).
He finished his collegiate career with 3,324 yards rushing and 11 touchdowns.
As a result of the signing, the Buccaneers released defensive tackle Justin Frick.
Darby's signing comes on the heels of fellow seventh-round pick Chris Denman's signing on Friday morning.