The Ghanaian forward has completed his medical and signed his contract ahead of the transfer deadline, paving the way for him to become a Derby player.
Fuseini had initially travelled to England on Monday to complete a move to Birmingham. However, the deal was called off at the last minute.
READ MORE: Union Saint-Gilloise forward Mohammed Fuseini close to Birmingham City move
Derby, who had been among the clubs interested in Fuseini before Birmingham won the initial race for his signature, moved quickly to revive their pursuit and have now secured the 24-year-old on a loan deal with an option to buy.
If Fuseini impresses at Derby, the club will have the option of signing him on a three-year deal.
Fuseini leaves Union Saint-Gilloise after a successful spell in Belgium. Since joining the club in 2024, he helped them win the Belgian Pro League title and Belgian Cup and also featured in the Champions League.
He made a strong start to the current campaign, registering two assists before scoring a spectacular curling effort against Bodø/Glimt in Champions League qualifying.
The forward was left out of Union's squad for Sunday's 3-0 victory over Anderlecht as his departure was being negotiated.
