The 31-year-old spent the last four months of the season on loan at the Championship club, who missed out on a place in the promotion playoffs after finishing the campaign badly despite Bent's 12 goals in 17 appearances.
“Darren Bent is a proven goal-scorer and that was something he proved during his loan spell with the club last season,” Derby's new head coach, Paul Clement, who has moved back to England from Real Madrid, told the club website (www.dcfc.co.uk).
“He is going to be key in our plans to move forward.”
The former England striker cost 18 million pounds when he joined Villa from Sunderland in January 2011.
Bent netted on his debut and finished as joint top scorer that season but from 2013 played much less frequently.
He spent most of the last two campaigns on loan at Fulham, Brighton & Hove Albion and then Derby, who have also announced the signing of Ireland international defender Alex Pearce from Reading.
Original source: Bent joins Derby after leaving Villa