While the trend is rather a disaffection of saunas, Paris is fairly well equipped with quality saunas. As in sex clubs, the naturism is trendy and hygiene is well respected in the most high range saunas. No foam evenings or decadent parties by poolsides... there are also decadence spots for that; -)
The most well-known is often Sun City (62 boulevard de Sébastopol, Etienne Marcel metro) because it is central, vast and original with its false Indian decor, its large swimming pool, jacuzzi, its very large hammam with many little corners, caves with showers, the large workout room... Upstairs, a labyrinth with its erotic paintings, cabins with wooden doors and some dark areas. It is quite crowded with sexy guys. The only drawback being that the clientele is more inclined to walk and watch than to get to the physical act.
There is IDM (4 rue du Faubourg Montmartre, Grand Boulevard metro) which is also very spacious with 4 lenghthy levels. The decor is not as amazing as at Sun City and there is a little less equipment, but it is perhaps more sex and it is 100% naturist from Thursday to Sunday. Some cabins are themed and of very different size... the place is to discovered and is full of hidden corners.
Key West (141 rue Lafayette, Gare Du Nord metro) also opted for 100% naturism day but from Monday to Wednesday. It is on 3 very pleasant levels with an important black clientele. It is very lively at after-office hours with people going before taking a train at the nearby Gare Du Nord. The hammam is hot, with no shyness, but further action happens in cabins.
The Gym Louvre (7 rue du Louvre, Louvre-Rivoli metro) is a gym which covers the whole ground floor. There is a large hammam that can be busy, cabins and a labyrinth. The clientele is rather young and muscular although some come here to hang out in the hammam more than in the gym.
In an oriental style, Le Riad (184 Rue des Pyrénées, Gambetta metro) is a very small sauna that is quite charming. It is a little isolated and not always busy, but really friendly and with a nicely designed decor.
Otherwise, there is King Sauna (21 Rue Bridaine, Rome or Place de Clichy metro) which is rather a popular neighbourhood.
Then come the other saunas frequented by a mature clientele: the kitchissime but well-maintained Bains d’Odessa (5 rue d'Odessa, Montparnasse metro) installed in historic baths built in 1895, which also becomes a spot for bears on Saturdays; Euro Men's Club (10 rue Saint-Marc, Bourse or Grands Boulevard metros) inspired by ancient Rome; the dark and aging TiL’T Sauna (41 rue Sainte Anne, Opera metro); Atlantide Sauna (13 rue Parrot, Gare De Lyon metro) and its bisexual clientele; the crumbling 100% naturist sauna Mykonos (71 rue des Martyrs, Pigalle metro) and finally the Bains Montansiers (7 Rue de Montreuil, Château de Vincennes metro) and its bears clientele.