This chapter reviews the current knowledge of the marine (including deep-sea species) and freshwater fishes of the Gulf of Guinea oceanic islands. Some biogeographic and conservation considerations are also presented. A total of 1045 species are likely present in the region, including 107 Elasmobranchii (37 confirmed, 65 potential, and 5 erroneous), one confirmed Holocephali, and 937 Actinopteri species (515 confirmed, 385 potential, 32 erroneous, and 5 questionable). Most of the coastal species are shared with the surrounding African continental shelf, while several species are amphi-Atlantic (present in both sides of the Atlantic Ocean), and some species have sister-species in the western Atlantic. A total of 15 species are endemic to the region, and 2 are introduced. Further studies are still needed to better understand the ichthyofauna of the Gulf of Guinea oceanic islands and help policymakers better define conservation and protection plans.