The 24-year-old full back has agreed a three month deal which will keep him at Bramall Lane until 30th May, enough time to play in Play-Off Final should United get there.
United face Grimsby Town tomorrow night at Blundell Parkand is expect that he will start the game, however Curtis is not cup-tied but due to the timing of the signing he will not be able to play against Leeds United on Sunday.
"The opportunity came along to sign John and I had to make a quick decision at the weekend because Crystal Palace were interested," Blades boss Neil Warnock told the club's official website.
"I haven't guaranteed him a first-team place but he's got a great attitude and can play both full-back or centre-half which is a great help."
"Sheffield United are playing higher profile games, in the play-off race, it is just 45 minutes from my home in Manchester and I believe it is just a much better footballing move for me," said Curtis.