Hidalgo took the plunge around 10:00 CET on a glorious summer's day in Paris, with visitors crowding on nearby bridges to catch a glimpse of her after several postponements due to heavy rain and doubts about water quality.
Hidalgo, clad in a wetsuit and goggles, was joined in the Seine by Tony Estanguet, the head of the Paris Olympics Organising Committee, among others. At first she paddled and then swam front crawl with her face in the water.
"We have worked very, very hard and then you go down into the water and it seems natural," Hidalgo said after the swim. "The water is very, very good, a little bit cool."
The triathlon and marathon swimming legs of the Olympics, which run from July 26th to August 11th, are due to be held in the Seine.
According to the most recent July 12th Seine water quality bulletin, based on the Eau de Paris water analysis, the water quality would be suitable for swimming in six out of seven days at the Olympics swimming sites.
Decisions on whether to run Olympics events will be taken the night before, and early that same morning, with a technical committee including athletes, international federation, regional authorities and Meteo France making the call.
"The first athletes are arriving tomorrow, and so this is a very important message that, finally, the Seine is swimmable, and the triathlon and swimming events can be held here," Estanguet said.
Paris has been working on cleaning up the Seine so that people can swim in it again, as was the case during the 1900 Paris Olympics. Former Paris Mayor Jacques Chirac in 1988 promised he would swim in the Seine "in the presence of witnesses", but his plunge never materialised.
The city has built a huge storage basin capable of holding 46,000 cubic metres of waste water before it flows through a tunnel to a treatment plant. When the water meets the required health criteria, it will then be poured into the Seine.
If the river is not deemed to be suitable, organisers have contingency plans: the marathon swimming event will take place at Vaires-sur-Marne, where the rowing and canoeing events are held, and the triathlon will be turned into a duathlon.
Jenn Fluet, a 21-year-old tourist visiting from New York, said Hidalgo was brave. Asked if she would follow suit, Fluet said: "Hell no! It's dirty."
Quentin Mazars, a 33-year-old swimming club member who joined Hidalgo in the Seine, said he "was careful not to swallow any water".
Pierre Suzeau, a 66-year-old member of an outdoor swimming group, emerged from his dip energised.
"We are very happy to finally see swimming in an urban environment become a reality," he said. "We hope that the Seine and the canals will soon all be swimmable."
French Sports Minister Amélie Oudéa-Castéra had already taken a swim in the river on Saturday.
