The arrival of the 20-year-old, who played nine games for the R’s, helps to further plug the gap following the big-money sale of Alfie Mawson to Swansea City, with the Reds having also replenished their defensive ranks with the signing of Adam Jackson from Middlesbrough.
Reports in London suggest that the fee for Kpekawa is in the region of £450,000.
Following the addition of Kpekawa, Reds head coach Paul Heckingbottom is now set to focus his energies on completing the signing of Wigan midfielder Sam Morsy to boost his numbers in the middle of the park.
On his latest capture, Heckingbottom said: “We have monitored Cole for quite some time now and I am delighted to bring him in.
“He’s tall, quick, strong, with a good left foot. He’s only young and that will help him fit into this group of players.
“He’s ambitious and wants to be here, that was key. He’s demonstrated all the ability and determination to be a Championship player and I can’t wait to get him integrated within the group, show him our system of play and get him out onto the pitch.”
Kpekawa has made one appearance for the R’s this season - in last month’s EFL Cup game against Swindon.
The Blackpool-born player, highly-rated during his time coming through the ranks at Loftus Road, has also had loan spells at Portsmouth, Colchester and Leyton Orient.
His arrival is the third signing of deadline-day week at Oakwell after the additions of Jackson and Adam Armstrong on loan from Newcastle United.