Fuseini has travelled to England and is scheduled to undergo his medical on Monday before completing the formalities of his transfer and signing his contract with Birmingham.
The deal gives Birmingham the opportunity to assess the 24-year-old over the course of the season before deciding whether to exercise the option to keep him at the club for three years on a permanent deal, Flashscore understands.
Birmingham moved quickly to win the race for Fuseini after interest from several clubs, with Troyes, Lorient and Derby County also exploring a deal for the Ghana international during the summer.
The Championship side ultimately emerged as Fuseini's preferred destination as they look to strengthen their squad for a promotion push.
The move comes after Fuseini was left out of Union Saint-Gilloise's squad for Sunday's 3-0 victory over Anderlecht.
Following the match, Union head coach David Hubert acknowledged that the Ghanaian could be on his way out of the club, adding further indication that his departure was imminent.
Fuseini had started the season positively for the Belgian champions, contributing two assists before scoring a spectacular curling effort against Bodø/Glimt in the Champions League qualifying rounds.
The Ghanaian joined Union Saint-Gilloise in 2024 from Austrian side Sturm Graz after impressing during a loan spell at Danish club Randers.
In his first season with Union, Fuseini helped the club win the Belgian Pro League title, ending a 90-year wait for the championship. He also featured in the UEFA Champions League as Union returned to Europe's elite competition.
His time in Brussels also ended with another major honour as Union lifted the Belgian Cup, giving Fuseini three significant trophies during his time with the club.
Fuseini arrives at a Birmingham side that have made an unbeaten start to the new Championship campaign.
They have five points from their opening three league matches and strengthened their promotion credentials with a 2-1 victory at Wrexham over the weekend.
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