The 24-year-old is eligible to make his Rams debut in Saturday's npower Championship clash with Scunthorpe.
Manager Nigel Clough told the club's official website: "We've been on the hunt for a striker with pace who can be a real menace to opposition defences for some time.
"He can get about the pitch and he will provide a goal threat for us.
"He will bring a different dimension to our play which we've probably not had all season."
Ward enjoyed successful spells with Torquay and Chesterfield before the Blades paid ?300,000 for his services in 2009.
Source: PA
Source: PA