The Republic of Ireland international keeper rejected the club's offer of a new deal and will depart as a free agent after two years at the club.
"Unfortunately Darren has informed us he will be leaving and naturally we are sorry to see him go," manager Gary Rowett told the club's official website.
"He has been an excellent player for us, an integral part of the team that finished tenth this season.
"Success breeds success and players do get other opportunities put before them which they feel they have to take.
"Darren leaves with our very best wishes and our thanks for his contribution.
"As a Club we now move on and we look elsewhere."