Soon after claiming victory, Gregoire, 48, took a city bike through the streets of Paris toward City Hall, echoing his promise to make the French capital greener.
Gregoire beat out prominent conservative contender Rachida Dati, who acknowledged her defeat after partial results were made public.
The race for Paris mayor was part of France's second round of municipal elections. Sunday's vote showed clear gains for the traditional left and right, and one major win for the far right in the French Riviera city of Nice.
Hidalgo was elected in 2014 and reelected in 2020 but chose not to seek a third six-year term after leading the city through the 2015 extremist attacks and the 2024 Paris Olympics.
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