There are eight official naturist beaches in Portugal currently. From to south north, they are:

LISBON:
Bela Vista and Praia da Adiça – Costa da Caparica, Almada
Meco – Alfarim, Sesimbra

ALENTEJO:
Salto – Porto Covo, Sines
Alteirinhos – Zambujeira do Mar

ALGARVE:
Adegas – Odeceixe
Ilha Deserta – Faro
Barril – Ilha de Tavira

There are plenty more where nudity is accepted However, or where all or part of the beach has become naturist in practice. The following list is by no means definitive – within reason you can go naked on any stretch of quiet beach, and Portugal has over 1800km of coastline – but it’s a good starting point.

PORTO & THE NORTH:

Afife, Rodanho, Estela, Rio Alto, Moreiro, Cabedelo, Furadouro, Costa Nova, Duna Alta, Palheirao, Almadoiros.

SILVER COAST:

Cova-Gala, Osso da Baleia, Vigao, Pedras Negras, Norte, Salgado, Gralha, Rei Cortico, Praia d’el Rei, Almagreira

LISBON REGION:

Valmitao, Aguda, Adraga, Ursa, Abano, Fonte da Telha, Rio da Prata, Bicas, Ilhau

ALENTEJO:

Comporta, Brejos, Aberta Nova, Monte Velho, Areias Brancas, Pessegueiro, Malhao, Brejo Largo, Amalia

ALGARVE:

Amoreira, Vale Figueira, Bordeira, Murracao, Barriga, Cordama, Beliche, Barranco, Zavial, Furnas, Cabanas, Pinheiros, Meia Praia (currently waiting final approval as an official naturist beach), Prainha, Caneiros, Afurada, Praia Grande, Armona, Cacela Velha