Joint winner of the Bormio classic earlier this season, the 23-year-old Paris outshone the favourites in one minute 57.56 seconds to emulate compatriot Kristian Ghedina, crowned in 1998.
The victory underlined the strength of the Italian speed team, who have bagged four of the six downhills held this season, with Christof Innerhofer winning in Beaver Creek and in Wengen a week ago.
Innerhofer was the focus of attention for the 50,000 spectators lining the Streif on Saturday, as he started in 46th position after being sanctioned for ignoring an order to slow down in practice.
As visibility worsened, the super-G world champion finished 21st, 1.92 off the pace.
Erik Guay was second, 0.13 seconds adrift, the best result for a Canadian on the Streif piste in 30 years.
Hannes Reichelt salvaged Austrian pride by finishing third, 0.36 behind the winner.
Aksel Lund Svindal, the fastest in practice and winner of Friday's super-G, looked a clear favourite at the start but could manage only 10th place, 1.16 back.
Perhaps unsettled by the crash of Frenchman Johan Clarey, who started ahead of him, the Norwegian surrendered the downhill World Cup lead to Paris by six points while losing vital ground in the race for the overall title to Austria's Marcel Hirscher.
Hirscher will be the favourite at home in Sunday's slalom here.