Trierweiler, 48, was admitted to a Paris hospital Friday, after she read in Closer magazine that Hollande had been spending nights with actress Julie Gayet, Britain's the Telegraph reported.
She was admitted for rest and to get tests done, her office said.
Hollande, 59, has not addressed the claims but said they are a breach of privacy. He is expected to discuss the alleged affair at a news conference, set for Tuesday.
Trierweiler, a journalist, has not publicly addressed the claims, and Gayet has also remained silent.