The morphology, 16S rRNA gene phylogeny and 16S–23S rRNA gene ITS secondary structures of three strains of marine Cyanobacteria, isolated from inter- and subtidal environments from north Portugal were studied, resulting in the description of Zarconia navalis gen. nov., sp. nov. (Oscillatoriales incertae sedis), Romeriopsis navalis gen. nov., sp. nov. (Leptolyngbyaceae) and Romeriopsis marina sp. nov., named under the International Code of Nomenclature for algae, fungi, and plants. No diacritical morphological characters were found for the new genera and species. The 16S rRNA gene maximum-likelihood and Bayesian phylogenies supported that the genus Zarconia is a member of the Oscillatoriales, morphologically similar to the genera Microcoleus and Phormidium, but distant from them. The genus Romeriopsis is positioned within the Leptolyngbyaceae (Synechococcales) and is closely related to Alkalinema. The secondary structures of the D1-D1′, Box B, V2 and V3 helices corroborate the phylogenetic results. Furthermore, our study supports previous observations of polyphyletic Oscillatoriales families and reinforces the need for their taxonomic revision.