The move follows an impressive loan spell at Brammall Lane at the end of last year, scoring 3 goals in 4 games. He will resume his United career in the Sheffield derby tomorrow.
The 33-year-old said: "I made it clear I wanted to sign, Neil Warnock knew that and he pulled up trees to get me. I've been ringing him constantly, pestering him to sign me.
"I want to play first team football. I'm delighted to join a First Division team doing well, hopefully now I will be able to score a few goals that can guide us towards promotion."