The Tykes have announced that the 19-year-old is to see out the season at Oakwell.
Knight was released by Newcastle in January, but has been offered an opportunity to earn himself a long-term deal at a Championship club.
It is unlikely that he will figure for Barnsely's first team before the season is out, as they battle to avoid the drop, but he will get the chance to impress the club's coaching staff.
Barnsley endured a frustrating evening on Thursday as the loan window slammed shut, but they are pleased to have drafted in a player of potential.
Tykes director Don Rowing said in The Star: "We've signed Dennis Knight until the end of the season.
"He's joined us on non-contract terms and will probably play for our U21 side to get some games under his belt."