The controversial 33-year-old striker, who left Birmingham by mutual consent in August, has signed a deal that will keep him at Bramall Lane until the end of the season.
Blades manager David Weir told the club's official website: "Marlon is vastly-experienced and has scored goals throughout his career, many of which that have come in the Championship
"We have been lacking someone with his experience and know-how and his presence will take the pressure off the younger lads, while giving everyone a lift.
"Marlon has recently played international football so although he hasn't played at club level this season we will be looking to introduce him as quickly as possible."
The Jamaica international could be included in Weir's squad for Saturday's home game against Preston as the Blades bid to end a run of four straight league defeats and secure their first win since the opening day of the season.
King began his career with Barnet in 1998 and has scored a total of 166 goals in 492 senior appearances.
His previous clubs include Gillingham, Nottingham Forest, Leeds, Watford, Wigan, Coventry and Birmingham, while he has also had loan spells with Hull and Middlesbrough.
King has hit the headlines for the wrong reasons on numerous occasions throughout his career and in 2009 was sentenced to 18 months in prison for sexually assaulting and punching a 20-year-old female student.
He was released from prison in July 2010 after serving half his sentence and was also arrested in connection with a hit-and-run incident that left a man injured in July of this year.