Mr Sarkozy is accused of accepting thousands of euros from L’Oreal heiress Liliane Bettencourt, now aged 90.
The former leader denies taking financial advantage of Mrs Bettencourt.
His lawyer said he would file an appeal against the “incoherent and unfair decision”, AFP news agency reports.
Magistrate Jean-Michel Gentil, who leads the inquiry, unexpectedly summoned Mr Sarkozy for a face-to-face encounter with Mrs Bettencourt’s butler, Pascal Bonnefoy, in the city of Bordeaux.
The judge wanted to determine how often the politician had met Mrs Bettencourt in 2007.
While Mr Sarkozy has maintained he only saw her once during that year, the butler gave a different account on Thursday, the BBC‘s Christian Fraser, in Paris, reports.
Following the hearing, prosecutors said the ex-president had been placed under formal investigation “for taking advantage of a vulnerable person during 2007 to the detriment of Liliane Bettencourt”.
Under French law the court’s decision falls short of a formal charge, our correspondent says.
Investigators will press ahead with the enquiry before deciding whether he should face a trial.
The former president previously hinted that he was considering another tilt at the presidency in 2017. The outcome of the investigation could determine whether he will make a return to politics, observers say.
Police raided Mr Sarkozy’s home and offices last July after he lost his presidential immunity.
He was declared a material witness in November, which meant he was a suspect but had not been formally charged.
Mr Sarkozy met Mrs Bettencourt when he was mayor of the wealthiest suburb in Paris and forged a close friendship with her over the years, our correspondent says.
He was a regular visitor to the family mansion, according to her staff.
It is alleged that staff acting for Mrs Bettencourt gave 150,000 euros (£120,600) in cash to Mr Sarkozy’s aides during his successful 2007 campaign to become president.
Individual campaign contributions in France are limited to 4,600 euros.
Mrs Bettencourt’s former accountant, Claire Thibout, has alleged Mr Sarkozy’s campaign treasurer at the time – Eric Woerth, who later became budget minister – collected the cash in person.
She also revealed in a leaked police interview that Mr Sarkozy, while mayor of Neuilly from 1983 to 2002, paid “regular” visits to the Bettencourt house.
But Mr Sarkozy has dismissed as mere gossip claims that he took envelopes stuffed with cash.
“[The Bettencourt] never gave me a single penny and I never asked them for any,” the politician was quoted as saying by the Sud-Ouest newspaper.
Mr Woerth, who was forced to resign as UMP party treasurer in July as a result of the scandal, is already under formal investigation over the 150,000 euro payment allegations.
The allegations surrounding Mr Sarkozy and Mr Woerth first surfaced in connection with a trial over Mrs Bettencourt’s estimated 17bn euro fortune.
Mr Woerth denies any wrongdoing, as does Mrs Bettencourt.