The 29-year-old, who had a loan spell at neighbours Sheffield United 10 years ago, has been training with the Owls for the last few weeks, and played in a behind-closed-doors friendly for them.
Having seen that, manager Darren Moore has decided to make the move permanent.
"It was lovely for him that he wanted to play in the game and blow off some cobwebs and he looked really good," said Moore.
The winger was sacked by Cardiff City after it was revealed he took cocaine the night before a match in July 2020.
But former Bluebirds manager Neil Warnock took him to the Riverside in February and he made two starts and seven substitute appearances for Boro, scoring once, before his short-term contract came to an end. Since then he has been without a club.
Of his 92 appearances for Cardiff, 20 came in the Premier League having been part of their promotion-winning side.
He began his career at Wolverhampton Wanderers, who loaned him to the Blades and Peterborough United, who he joined permanently. There have also been loans at Portsmouth, Shrewsbury Town and Cambridge United, plus a permanent spell at Rochdale.
The Owls are currently playing with a wingerless 3-5-2 formation, but Moore has made it clear he is not wedded to any one system, and is fond of trying players in different positions.
The club were hoping to hear by 5pm today if another free agent - a centre-back - had taken up their contract offer.