Allen, a native of Leesburg, Va., was second in the SEC and tied for seventh nationally with 12 sacks. He recorded 36 total tackles while leading the Tide with 14.5 tackles for loss.
By retaining Allen, the Tide's defensive line returns a two-year starter who can rush the passer from the edge and inside while also holding up against the run.
Allen joins outside linebackers Tim Williams and Ryan Anderson and safety Eddie Jackson as draft-eligible players returning to school for their final seasons.
Heisman Trophy winner Derrick Henry and defensive tackle A'Shawn Robinson both declared for the draft on Friday afternoon.
Tight end O.J. Howard has yet to make his intentions public.