A cyanobacterial strain isolated from Brazilian mangrove soil and classified as a member of the genus Phormidium was afterward affiliated to the genus Oxynema . To define the species of this Oxynema strain CENA135, we sequenced its whole genome and applied a polyphasic taxonomic approach. This strain, with all the morphological features recognized for the Oxynema genus, had its genome assembled in 11 scaffolds with a total size of 6,235,022 bp, a G + C content of 51.6%, 4,720 protein-coding genes, and five 16S rRNA genes. Genes related to ecological resistance were annotated, demonstrating the relevance of obtaining high-quality genome assemblies from underrepresented habitats. A phylogenomic tree inferred by GTDB-Tk based on the alignment of 120 conserved proteins clustered Oxynema sp. CENA135 together with the strain Oxynema aestuarii AP17 isolated from Indian mangrove soil, and digital DNA-DNA hybridization and average nucleotide identity values between these two strains were 92.8 and 95.78%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences placed the strain CENA135 in a separate and well-supported major clade (100% bootstrap) containing Oxynema species, and its 16S rRNA gene sequence showed identity ≤98.6% compared to the other species of the genus. Moreover, the strain CENA135 had a distinct 16S-23S ITS sequence and secondary structure polymorphisms in comparison to the other Oxynema species, supporting its recognition as a novel species. On the basis of evidence from this polyphasic study, strain CENA135 should be designated as representing a novel species of the genus Oxynema , for which the name Oxynema mangrovii sp. nov. is proposed under the provisions of the International Code of Nomenclature for algae, fungi, and plants.