The Seagulls on Wednesday confirmed the 24-year-old, who joined them last month from Mamelodi Sundowns for an undisclosed fee, will spend the 2018/19 camapign in the Belgian second division.
"Brighton & Hove Albion striker Percy Tau has joined Belgian side Royale Union Saint-Gilloise on a season-long loan deal," read a statement on the Premier League club's website.
Brighton's 'development pathway' manager David Weir has since explained that the club will continue to track the South African during his time in Brussels, with the hopes that he will gain European experience prior to his return to England.
“Percy is a talent we have brought to the club for the future, and this loan allows him to play regular senior football here in Europe," said Weir.
“It is a significant investment for us, and we will be closely watching his progress with Union and monitoring his development over the season.”
Saint-Gilloise was taken over by Albion's owner-chairman Tony Bloom as the majority shareholder in May this year.
Tau had been training with the club in recent days prior to his move being confirmed.