The present publication is a new annotated and updated checklist of the opisthobranchs (Mollusca, Gastropoda) from the Spanish and Portuguese coasts, including their Atlantic archipelagos (Azores, Madeira, Selvagens and Canary Islands). The bathyal species recorded from the continental shelf of all these areas are also included. Incorporating a review of the literature, 523 species are included on the present checklist, 23 belonging to Architectibranchia, 111 to Cephalaspidea s. s., 14 to Anaspidea, 4 to Acochlidiomorpha, 37 to Thecosomata, 7 to Gymnosomata, 43 to Sacoglossa, 3 to Umbraculoidea, 16 to Pleurobranchoidea and 265 to Nudibranchia: 127 Doridoidea, 42 Dendronotoidea, 9 Arminoidea, and 87 Aeolidoidea. The records these species have been divided into 12 geographic sectors: 1) Spanish coast on the Bay of Biscay; 2) Galicia; 3) mainland coast of Portugal; 4) Andalusian Atlantic coast; 5) Straits of Gibraltar, including Ceuta (northern Africa); 6) Andalusian Mediterranean coast, ...