The 30-year-old also had a loan spell with the Tykes last season when he scored three goals in nine games from November 14 to January 1 before a broken collarbone ended his stay at Oakwell.
Tudgay signed a new one-year contract at the City Ground during the summer but has yet to feature for Forest this season.
"I jumped at the chance. They're a great set of lads here," Tudgay told Barnsley's official website.
"I always say that given the opportunity, I will score goals. If I can come here, score one or two goals and help the team then that'll be good."
Former Derby and Sheffield Wednesday striker Tudgay will wear the No 50 shirt for Barnsley and is likely to be involved in Saturday's home game against fellow Championship strugglers Birmingham.