BFM Television said the Elysee presidential palace had confirmed that Trierweiler had been hospitalized since Friday because of exhaustion.
French daily Le Parisien said earlier on its website that Trierweiler, shaken by the revelations about the affair, had been taken to hospital on Thursday, right after hearing about Hollande's liaison.
The paper said doctors had prescribed rest and that she would be discharged in a few days.
Trierweiler's spokesperson was not immediately available.
On Friday, Hollande threatened to sue celebrity magazine Closer, complaining of breach of privacy after it said he was having an affair with actress Julie Gayet.
Hollande and Trierweiler, a journalist for celebrity magazine Paris Match, are not married but have been in a long-term relationship for several years.