The Blades have completed a fourth signing of the January transfer window.
Ricky Holmes arrives at Bramall Lane on a two-and-a-half-year contract after an undisclosed fee with Charlton Athletic was agreed.
Holmes, 30, is United's third permanent signing after the captures of Ryan Leonard and Lee Evans, whilst James Wilson, who made his debut in the Steel City derby on Friday, is on loan for the remainder of the season.
Blades boss Chris Wilder has succeeded following a lengthy pursuit of attacking player Holmes.
"I worked with Ricky previously and it is no secret that I'm an admirer of his ability," said the United chief.
"He has something a little different and I'm confident that his best years are ahead of him. He completes the jigsaw of our incoming transfers and I must thank Paul Mitchell (head of recruitment) and Carl Shieber (head of football admin) for securing these deals early in the transfer window."
Holmes progressed through the non-league route into the professional game and played under Wilder's leadership at Northampton Town.
He has also played for Portsmouth and, latterly, Charlton, for whom he scored 19 goals in 58 league appearances, including one at Bramall Lane last season.