The Blues have officially confirmed that the deal has been cut short through mutual consent.
The American did manage to play 14 games for the League One team, but now has to find a new team.
Slonina was a starter when he first went to the lower league side, but then lost his place.
Chelsea are likely to pursue a new loan for the young shot stopper, as they want to continue his development.
“Barnsley fans, it comes to an end,” he wrote on social media.
“It wasn’t the way I would’ve liked the first half of the season to go, but I thank you for welcoming me into the team and allowing me to be a small part of what I believe will be a successful season for you all. A big thank you to the staff and players for making me feel part of this club.”
He continued, “Being in and out of the environment, I know how hard everyone’s working behind the scenes to ensure performances meet the expected standards. I wish you all the best moving forward; Barnsley will always hold a special place in my heart. And of course, enjoy your big win this afternoon.”