In theory, nudism is perfectly legal throughout Spain. I saw two groups of people walking through central Barcelona, completely starkers in a single week last year. Which means that any beach in Spain can be a nudist beach. However, in practice nudists congregate in particular beaches, usually those slightly out of the way.
Platja Balmins, is a mixed nudist beach, between Sant Sebastià and the port of Aiguadolç. There is another nudist beach at Cala del Home Mort (Dead Man's Beach), which is mainly gay. Further up the coast, in Vallcarca, to the north of Sitges is a third naturist beach, Cala Morisca.
Has the advantage of being right in the middle of the city of Barcelona. Walk along the main Barceloneta beach and the nudist section is behind some dunes, after the nautical club.
Who said nudists have to like the beach. If the mountain air is more your thing, try Sierra Natura, near Valencia. Alternatively, there is El Fonoll in Tarragona.
Naturist resorts aren't restricted to mainland Spain - Lanzarote in the Canary islands has this fine collection of swimming pools and villas for nudists.
Cabo de Gata is an unspoilt area of beaches in the south-east of Spain. Hippy communes, nature parks and plenty of space to strip off. Media Luna, Cala Carbón and Cala del Barronal are just some of the nudist beaches in this area.