Qatar Sports Investments (QSI) said in a statement it had "completed the acquisition of KAS Eupen".
"This formalises QSI's full ownership of the club, following its takeover of KAS Eupen's sporting operations in December 2025," it added.
QSI and the Belgian club "have been working closely together to build a strong foundation for the club's long-term sporting and commercial development", the statement said.
The Qatari investors acquired a majority stake in PSG in 2011 and have held a minority stake in Portuguese club Braga since 2022.
Since acquiring PSG, the French club has won 12 of its 14 Ligue 1 titles and is the two-time defending Champions League winner.
Eupen have been under Qatari control since 2012, but through the Aspire Academy, which uses the club as a springboard for young talents.
The club were relegated to the Belgian second division in May 2024 after eight seasons in the top flight.