The Ostreidae are well known for their high intra-specific variation, which makes identification problematic. The present paper aims to provide a morpho-anatomical and taxonomic review of the Brazilian species of Crassostrea, as well as some congeneric species from other relevant areas. The Brazilian species are Crassostrea mangle sp.nov. and Crassostrea brasiliana. The exotic species Crassostrea gigas, cultivated in the southern region of the country, is also included in this study. Additional species are: Crassostrea virginica, from the Atlantic coast of the USA, the type species, and Crassostrea rhizophorae, a south-eastern Caribbean species that is supposed to occur in Brazil, but is here understood as a different entity endemic to that region. Taking into account their economic importance, the differentiation between these species is critical, and is essential for a better planning of production and preservation strategies. We point out the differences in almost all structures amongst these species, which are formally redescribed herein. The occurrence of the African species Crassostrea gasar in Brazil is established as false. The voucher specimens of previous studies responsible for this assumption were examined, evidencing a misidentification for Crassostrea brasiliana from an estuarine environment. The geographical distribution of the studied species is recognized as follows: Crassostrea rhizophorae in the south-east Caribbean Sea; Crassostrea mangle sp. nov. from Pará to the Santa Catarina, only in mangroves; Crassostrea brasiliana from Paraíba to Santa Catarina, both in rocky shores and mangroves; Crassostrea virginica ranges from the Atlantic coast of Canada to the Caribbean; and Crassostrea gigas originates from the Indo-Pacific, but has been introduced in southern Brazil. This paper also deals with conchological aspects of the endemic species Crassostrea praia, from south Lagoa dos Patos, Rio Grande do Sul; after a more detailed definition, and considering the deleterious effects of the nearby port and construction sites, it can be classified as an endangered species.