"I've stepped down a league but this is a great chance to get straight back into the Premiership," Horsfield told the official Sheffield United website.
"It has been in the papers for over a month that United wanted to sign me.
"I'm delighted it is done and that I can come back up to Yorkshire and complete the deal.
"My aim now is to get a few goals to help us to promotion.
"I've only been here about an hour and I've seen the size of the club and we deserve to be in the Premiership."