A new filamentous cyanobacterial species of the genus Brasilonema was isolated from the island of Oahu, Hawaii. The taxon is distinguished from the seven other species in the genus by attenuation of trichomes, and is here described as Brasilonema angustatum sp. nov. It possesses the cytoplasmic kerotimization and reddish-brown coloration of several species in the genus. Phylogenetic analysis based on 16S rRNA gene sequence shows B. angustatum within a highly supported clade containing all sequenced Brasilonema species. We compared the secondary structure of the 16S–23S ITS regions for B. octagenarum and B. angustatum. The structurally conservative D1–D1′ and V3 helices show similar motifs between the two taxa, but differ structurally and in sequence, providing additional justification for erection of the new species. The Box-B helix has identical secondary structure. The existence of tapering in a Brasilonema species is unique in this genus, and requires modification of the current concept of the genus Brasilonema, which was described as being unattenuated. Our phylogenetic evidence supports the hypothesis that tapering has developed repeatedly in separate cyanobacterial lineages and lacks the taxonomic significance once assumed by early workers.