The time trial France champion, who finished in a small group 50 seconds behind the day's winner, Germany's Nico Denz, takes the overall lead with just under two minutes ahead of Britain's Geraint Thomas, who came far behind in the pack of favourites.
"It wasn't really the plan, the goal was to take the breakaway and aim for the stage win. I can't realize I'm taking the pink jersey, I can't believe it," he said, thumbs up and fist raised on the podium.
"It became a reality in the last five kilometers, before I didn't really think about it. For the team it's great and for me it's a dream," he added.
If a final victory is difficult to envisage for the Haut-Pyrénéen in this Giro, especially in view of the very mountainous third week that awaits the peloton, it is a great reward for the one who achieves a very successful Tour.
In addition to fulfilling his role as Thibaut Pinot's teammate, as he did the day before in the first pass, he finished fifth in the second time trial last Sunday.
The 29-year-old, who has not won any professional races apart from his national time trial, was 23rd overall on the morning of Saturday's stage, 18 minutes and 37 seconds behind Geraint Thomas.
No Frenchman had donned the pink tunic since 1999 when Laurent Jalabert wore the jersey for eight stages, before handing it over to Italian Marco Pantani.
Laurent Fignon remains the last Frenchman to have won the Giro d'Italia, in 1989, and the third French rider to win the Giro after Jacques Anquetil (1960, 1964) and Bernard Hinault (1980, 1982, 1985).
© 2023 AFP