Woolford, 29 has joined the Blades on a two year deal after leaving Millwall this summer following their relegation from the Championship. He leaves the Lions having scored 11 goals in 79 appearances for the New Den club.
He has become new Blades boss Nigel Adkins first signing for United and teams up with his former boss having helped Scunthorpe United to play-off success in 2009.
Adkins told sufc.co.uk; "Martyn is fit, strong, robust and honest and has a fantastic professional attitude. We are very aware of the talents Martyn brings to Bramall Lane. He has been consistently available for selection over many seasons and knows the demands required of this level."