Sheffield-born Forte has been out of favour at Southampton this season and will return to his first club where he will join fellow new-boys Danny Higginbotham and Jamie Murphy, who signed earlier in the month.
Boss Danny Wilson told the club's website: "People will be familiar with Jonathan's talents and we are delighted that he has accepted our offer to come and play.
"His goalscoring record is very good and hopefully he can improve that back in Sheffield with us - he is undoubtedly a player with great potential."
The 26-year-old came through the Blades Academy and made his debut in an FA Cup tie in 2004. He played 39 times for the first team before moving to Scunthorpe where he really made his name.
He followed boss Nigel Adkins to Southampton in January 2011 and in a frustrating two-years he was farmed out on loan to Notts County, Preston North End and Crawley Town.
Sheffield United have also completed the signing of Scotland international midfielder Barry Robson on a deal until the end of the season.
The 34-year-old was a free agent after his contract with the Vancouver Whitecaps was cancelled earlier this month by mutual consent.
Robson has played 17 times for Scotland and started his career with Inverness before moving on to Dundee United, Celtic and Middlesbrough.
Wilson said: "When someone of his experience and quality comes along you have to sit up and take notice and I'm delighted a deal can be completed, especially given the fact that he has been back in the country less than a week.
"He gives us options in offensive positions, he will score goals and be a supply line for others."