Maggetti pumped her fist as she crossed the line and leapt off her board into the Mediterranean in celebration.
The Italian's break away from her rivals in the third leg of four proved decisive, giving her the lead from Britain's Emma Wilson, who had dominated the early stages of the race, during which their iQFOiLs flew over the sea at more than 20 knots.
Israel's Sharon Kantor, who had been in second place for most of the high-speed chase between the three windsurfers around the bay off Marseille, secured silver.
Wilson, who had dominated the fleet during the qualifying stages, crossed the line some way behind her rivals. The Briton, who also won bronze in Tokyo, looked distraught.
Maggetti swam over to her rivals to hug them in the water as the result of the race began to register. She was greeted back on the beach with an Italian flag and ran over to embrace friends and family waiting for her.
There were emotional hugs ashore too for Wilson, whose tears were interspersed with occasional smiles.
