The 33-year-old, who was released by the Blades at the end of last season, will go straight into the squad for North End's opening-day clash with Colchester.
Manager Graham Westley said: "It has been a long summer looking for a goalkeeper after Richard Wright departed the club.
"We have seen a lot of goalkeepers and a number of good lads, but Steve fits the bill in all regards.
"I wanted an experienced goalkeeper, somebody who knows the game but still has the enthusiasm and the energy to make a contribution going forward.
"He has got a lot of class about him and a lot of experience, he is a good lad and I am looking forward to working with him."