Recent phylogenetic studies of the genus Pterosiphonia suggest that Pterosiphonia dendroidea and Pterosiphonia tanakae are conspecific and show its wide geographic distribution in the Pacific. Pterosiphonia dendroidea was originally described as Polysiphonia dendroidea from Peru and later transferred to Pterosiphonia. Pterosiphonia dendroidea was characterized by diagnostic features such as its light cortication, the presence of trichoblasts, 8-12 pericentral cells, branches with the production of 3-to 5-order laterals, and congenital fusion of 2-3 segments. Pterosiphonia tanakae shares these features. Our phylogenetic analysis using rbcL sequences reveals a low gene sequence divergence (0.5%-0.7%) between the samples of P. dendroidea and P. tanakae. We propose that P. tanakae is a later synonym of P. dendroidea, and our study indicates the wide distribution of P. dendroidea in the northern and the southern Pacific Ocean.