Liam Kitching has become Barnsley's first signing of the January transfer window.
The 21-year-old centre-back has joined from League Two Forest Green Rovers.
He came through the Leeds United academy with the likes of Aapo Halme and Clarke Oduor before moving to Forest Green Rovers in 2019. He has played 53 games for the League Two club.
Kitching will not make his debut against Tranmere Rovers on Sunday as he is cup-tied for the FA Cup.
The fee has been reported at £500,000. He has signed until 2024 with a club option to add an extra year.
Left-sided Kitching will provide competition for starting centre-backs Mads Andersen, Michael Sollbauer and Michal Helik. Aapo Halme and Jordan Williams can also play there but have both been injured.
The Reds are also hoping to bring in Norwich City striker Carlton Morris this week.