Neosiphonia japonica is a rhodomelacean red alga that occurs in Korea, Japan, China, far-east Russia, northwest America, and New Zealand. Although it is distinguished by a bush-like habit having four pericental cells with cortication and numerous branches on axes, the taxonomy of N. japonica is still problematic. To investigate the taxonomy and phylogeny of the species, we analyzed rbcL and phycoerythrin (cpeA/B) genes from 19 samples of N. japonica and putative relatives. Phylogenetic trees from both genes show that N. japonica from Korea, Japan, New Zealand, and USA is clearly separated from N. decumbens, N. harlandii, and N. flavimarina from the Pacific Ocean. Instead, N. harveyi from the Atlantic Ocean was more related to N. flavimarina than to N. japonica. This result supports morphological and distributional differences between N. japonica and N. harveyi. However, the close relationship between these species suggests that they might have a recent most common ancestor. This is the second report to use the cpeA/B gene for evaluating species diversity in the Rhodophytes.