A new subaerophytic cyanobacterium, Ewamiania thermalis gen. et sp. nov., was isolated from a thermal spring complex in tropical, north-eastern Australia and characterised using combined morphological and phylogenetic attributes. It formed blackish-green hemispherical caespitose mats that began as small circular tufts, maturing to form dense mats up to several metres long. It grew along the crests of the minidams just above the thermal waters as well as along some of the shallow unconfined areas of vent-discharge aprons. Morphologically, Ewamiania is most similar to members of the Scytonemataceae. Filaments were isopolar, cylindrical, straight or flexuous, densely arranged and erect, often parallely fasciculate, with tolypotrichoid false-branching, rarely with scytonematoid false-branching. Vegetative cells were short barrel-shaped or isodiametric, slightly constricted at the cross-walls, with granulated contents. Sheaths were firm, thick, lamellated, uncoloured to yellowish or darkly yellow–brown in colour, cylindrical and closed at the apex. Heterocytes were spherical or ovoid in shape, and occurred in both basal and intercalary positions, generally solitary, but sometimes up to two or three in a series, developing particularly at the base of branches. Reproduction occurred by the production of hormogonia by the formation of necridic cells; hormogonia were not constricted at cross-walls and often included terminal heterocytes. Phylogenetic analyses using partial 16S rRNA sequences obtained from a strain of E. thermalis showed that it formed a well supported monophyletic clade, sharing less than 94.3% nucleotide similarity with other cyanobacterial sequences, including putatively related taxa within the Scytonemataceae. It also formed a novel clade in the nifH phylogeny, which was associated with members of the genus Brasilonema M.F.Fiore, Sant’Anna, M.T.P.Azevedo, Komárek, Kastovsky, Sulek & Lorenzi.