Bent scored 10 goals in 15 Championship appearances while on loan at Derby during the 2014-15 season. With Villa no longer requiring his services, Bent agreed to sign with Derby on a permanent basis and also has the option of a third year.
"Darren Bent is a proven goal scorer and that was something he proved during his loan spell with the club last season," Derby coach Paul Clement told the club's official website. "He is going to be key in our plans to move forward."
The 31-year-old striker joined Villa from Sunderland for 18 million pounds in 2011, but only scored 21 goals in the 61 Premier League games he played across four seasons.