Sapin acknowledged late yesterday "making a comment" while placing his hand on the woman's back at a conference early last year - following two previous denials of a claim that he had tweaked her knicker elastic.
Sapin's admission came on the heels of a scandal that broke on Monday when eight women accused a deputy parliamentary speaker, Denis Baupin, of sexually harassing them over a number of years.
Prosecutors have opened a preliminary investigation into the claims made by colleagues of Baupin in the ecologist EELV party.
"There was no sexist or aggressive intent in my action, but the simple fact that I shocked the person in question shows that these words and actions were inappropriate, and I was and remain sorry."
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Sapin said he had immediately apologised to the journalist in question.
"In the minutes that followed (the incident), the journalist asked to see me face-to-face to tell me of her outrage. Of course, I apologised sincerely to her," he said.
"We are in a place of absolute libel... These are allegations that are totally false," he had told reporters.
The allegations first surfaced in April, when two other reporters published a book about the corridors of power titled "Elysee Off".
The authors, Stephanie Marteau and Aziz Zemouri, claim the unnamed journalist in Davos was bending over to pick up a pen when Sapin said: "Ah, but what are you showing me here?" and snapped the elastic of her unintentionally exposed panties.