Worried about an excess of flesh visible on the banks of the River Seine, Paris City Hall has banned thong bikinis, topless sunbathing and nudity at the summer sand-in-the-city event known as Paris Beaches, Le Parisien newspaper reported.
Violators will be fined euro38 (US$48), the report said.
Thongs and "monokinis" — or bottom-only bikinis — are common sights on France's Mediterranean and Atlantic shores.
Paris was pushed to forbid "indecent attire" because it could "provoke temptation and dangerous behavior along the bank of a river," Pascal Cherki, mayoral aide in charge of sports, was quoted as saying.
City officials responsible for Paris Beaches could not be reached for comment Saturday. It was unclear why the ban was only imposed this year, the fifth year of the popular event.
Mayor Bertand Delanoe inaugurated Paris Plages in 2002, filling sections of the left and right banks of the Seine with sand and installing spray misters, hammocks, parasols and other beach-style accoutrements.
The monthlong event attracted 3.8 million people last year, and this year has pulled in many seeking relief from record heat.