The 25-year-old, known to teammates as Tobi, joins the Premier League club fresh off scoring 41 goals and providing nine assists in 82 appearances during three seasons with the Belgian side, a spell that included Union's first league title in 90 years. He is expected to be available for Brighton's opening fixture against Aston Villa on 23 August.
David's route to the top flight has taken in five countries. He began playing youth football in Canada before spells in Croatia and the United States, then moved to Malta in 2022 to play for Valletta and Sirens. A switch to Estonian side Nomme Kalju followed, where 30 goals in 44 appearances persuaded Union Saint-Gilloise to sign him in 2024.
He has since become a fixture for his country too, scoring four goals in 14 caps for Canada, including a goal against Switzerland at this summer's World Cup, where he appeared as a substitute in four of the side's five fixtures.
"Promise is someone we have been watching for some time, he is an out-and-out striker who possesses outstanding physical attributes and a track record of scoring goals," said Brighton sporting director Mike Cave.
"His experience at international level and in European competition will hopefully prove valuable this season. At 25, he's entering his prime playing years and we're excited to see him continue to develop in our environment."
David becomes Brighton's latest addition to a squad already reshaped for a season that includes a return to European competition. The Seagulls will hope his physical profile and finishing record offer a different attacking option to the club's existing forwards as the campaign gets under way.
Union Saint-Gilloise, for their part, lose their top scorer having built much of last season's title challenge around David's goals, and will now turn to the transfer market themselves in search of a replacement before their own domestic season progresses further.