Anne Hidalgo finally made the move ahead of the Games, which begin next Tuesday, after months of waiting.
The mayor was forced to postpone his original plan to swim in the river last month when tests revealed the amount of feces present was ten times greater than permitted limits.
Swimming in the Seine was outlawed a century ago, but French officials are eager to show that their €1.4 billion investment to stop sewage breaches has made the waterways safe to swim in again.
Wearing a wetsuit and goggles, Ms. Hidalgo dove into the river beside the head of Paris 2024.
Amelie Oudea Castera, the minister of sports, Tony Estanguet, and top public official Marc Guillaume made the leap earlier this week.
But President Emmanuel Macron, who had earlier said he would accompany them, has not shown there.
During the Olympics, which run from July 26 to August 11, and the Paralympics, which run from August 28 to September 8, the triathlon and marathon swimming legs are slated to take place in the Seine near the Alexandre III bridge.
