The 21-year-old forward signs a three-year deal at Bramall Lane, ending a transfer saga that has lasted all summer.
The Blades have beaten off competition from Norwich City, Wolverhampton Wanderers and Sheffield Wednesday to sign the frontman, who rejoins his boyhood idols.
Sharp moved to Glanford Park from the Blades two years ago, finishing top scorer in both of United's League One campaigns.
The hotshot hit 32 goals as the Iron stormed to the League One title last season, breaking a club record set by the legendary Barrie Thomas.
Forte signed earlier in the day and the club claim he is part of the Sharp deal.