Violators will be fined $48 (about R350), said the report.
Thongs and "monokinis" - or bottom-only bikinis - are common sights on France's
Mediterrannean and Atlantic shores.
Paris was pushed to forbid "indecent attire" because it could "provoke
temptation and dangerous behaviour along the bank of a river," said Pascal
Cherki, mayoral aide in charge of sports.
City officials responsible for Paris Beaches could not be reached for comment
on Saturday.
It was unclear why the ban was only imposed this year, the fifth year of the
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 month-long event attracted 3.8 million people last year, and this year has
pulled in many seeking relief from record heat.
