Alock, 26 has signed on a free transfer after his contract with Peterborough ended last month and came to Bramall Lane to become Nigel Clough's eighth summer signing - give the Blades a real intent of their push for promotion next season.
"We are delighted to get Craig, who has experience of the top end of this division and also the Championship in recent times," Clough told sufc.co.uk; "A major advantage is he can play anywhere across the back four and that will be hugely beneficially through what is a very, very long season."