From a distance, this small island just to the south of Kos looks just like a volcanic cone jutting out of the sea. And, indeed, that is more or less exactly what it is. Here it is on Google Maps.
Frewin Poffley's guide, whilst enthusing about the "small island" atmosphere, gives Nissiros a round zero for nudism.
However Barefooters who visited the island in May 2007 sent the Captain a very promising report.
Take the road east out of Pali past the old spa and continue until the dirt road ends with a car park. Continue on the trail for a further 15 minutes or so around the headland and you will arrive at the beach. The beach is coarse dark sand and small pebbles backed by dunes. Barefoot reporters visited on four consecutive days and about 90% of people were naked. Although some were clothed when our Barefooters arrived they soon stripped off when they saw they were naked. It is possible to take naked walks. More naturist the further you go along the beach and male only at the furthest point. At the furthest end a lone male appeared to be living in a cave behind the beach. No shade or facilities but beautifully quiet and unspoiled. Some naturist campers on the beach. My reporters rated the beach 95%. This seems to be it here
More reports from Nissiros would be very welcome.
The Pachia Ammos (Παχιά Άμμος) beach, described in the report above from 2007, is on the east coast of Nisyros about 7 miles (10 or 11 km) along the coast from Paloi. The road ends close to a restaurant about half a mile from the beach. From there is an easy path that goes a little bit up the hill and then down to the beach. It is also possible to walk the rocky shore, although I do not recommend it.
I spent two weeks in Nisyros in July 2009 and I found the Pachia Ammos beach just beautiful. It is a little less than half a mile long. At the northern end of it, about 50% of the people were naked but south of the middle it was about 100% naturist. About half of the 100 or so guests (mostly young people and about equally many of both sexes) seemed to be staying there in tents; about half seemed to commute from Paloi or Mandraki as I did.