The wife of President Nicolas Sarkozy is to unveil details of her new mission at a news conference on Monday with the fund's executive director Michel Kazatchkine, the French presidency said.
The weekly l'Express magazine reported that Bruni, 40, has agreed to be a goodwill ambassador for the protection of mothers and children against AIDS.
Based in Geneva, the fund oversees hundreds of programmes in 136 countries through public-private partnerships that have raised more than 11 billion dollars.
Bruni, who married Sarkozy in February, expressed an interest in work to battle HIV following a visit to South Africa, which has one of the world's largest AIDS caseloads.
The supermodel-turned-singer has said she hoped her status as French first lady could be used to advance a worthy global cause.