The 19-year-old Slonina didn't play a competitive match for Chelsea in his first season at the club following his move from the Chicago Fire. But he made his senior international debut for the United States in a friendly against Serbia in January.
Chelsea recently signed goalkeeper Robert Sánchez from Brighton.
"Thank you for the warm welcome from the entire club and fans," Slonina posted on X. "Can't wait to share the pitch with all of you. Let's have a successful season."
Eupen are in fourth place in the Belgium's top league after two games.
"It is a unique opportunity for KAS Eupen to sign a goalkeeper with the talent and class of Gabriel Slonina for one season," Eupen general director Christoph Henkel said.
"After all, before his transfer to Chelsea FC, several top European clubs were also interested in signing him. We are looking forward to working with Gabriel Slonina and are confident that he will give our team support and help us to achieve the goals of this season.
"We cordially welcome Gabriel in the KAS Eupen team."
Information from the Associated Press was used in this report.