Parker, 24 made over 40 appearances for the Whites since breaking through the youth team in 2005 and has also had loan spells with Bradford City, Darlington and most recently Carlisle United this February.
A player who has been capped by England at various youth levels he is unwanted by Neil Warnock and following the end of his contract has been allowed to leave the Championship club.
Sheffield United boss Danny Wilson is in search of new players and a left-back to replace the released Andy Taylor and sees Parker as his ideal replacement - however the former Barnsley boss already has Marcus Williams and an injured Lescinel Jean-Francois on his books.