Cotterill impressed United when he had a 3 month loan-spell at the Lane at the closing of the last term. He arrives on a undisclosed fee while Spring is set to put pen to paper on a 1 year loan spell.
The Wigan midfielder got plenty of crucial assists and was eager to return back to the Lane. United's promotion claim has now been boosted enormously in many of Unitedites eyes.
Cotterill who is 20 years old has signed a three year deal at the Lane. Blades boss Kevin Blackwell said: "Bringing David here is something we have been trying to conclude since the end of last season. He did quite well feeding the balls in for Beattie and Sharp and now with Henderson on board, we need that kind of service.
"He is only 20 and is a full international. He has shown glimpses of the real potential that enabled Bristol City to sell him for £2m. David has set his heart on a move here, is prepared to look for a new house in the area and we are showing the same commitment to bring him in."
Matthew Spring meanwhile has joined from Blackwell's former club Luton Town. More on Spring's arrival shortly.
Also putting pen to paper is goalkeeper Justin Haber who is a international for Malta. The Haidari shot stopper signs a two-year deal.
Thoughts on all three players and more from Blackwell to follow.