Read more about Nudism in Spain:
Whether this means the police would be happy for you to walk in the nude down Las Ramblas is questionable (I have actually seen a pair of protesters walking naked in central Barcelona, but there were no police present to test their reaction to it). But what it does mean is that nudism is widely practiced on Spain's beaches.
Because of this, there are no 'nudist beaches' as such, though there are beaches that have come to be known as such through the behavior of the bathers.
However, the local government in Cadiz, Andalusia, has raised the possibility of fining people caught naked on their beaches, in a move which has raised a number of legal questions: Confusion & Outrage at Cadiz Nudist Ban.
Aside from this new legal challenge to nudism in Spain, the acceptability of going unclothed in certain situations may also make you reconsider whether you get naked in Spain. You can't just strip off on any family beach. The wrath of an irate Spanish mother is worse than any trouble you could get into from a policeman!
But even the most popular and busy beaches may have an area at one end where nudity is practiced.
Certainly, if a beach is secluded, nudity should be acceptable there.
If in doubt, follow what others around you are doing. Unless you arrive very early in the morning, chances are there will be someone already bathing on the beach where you are - their choice of clothing (or lack of) should help you decide whether it's OK to get naked.