A molecular phylogenetic study of red algal parasites commonly found in the Northwestern Pacific and the Hawaiian Islands was undertaken. Four species, Benzaitenia yenoshimensis Yendo, Janczewskia hawaiiana Apt, J. morimotoi Tokida, and Ululania stellata Apt et Schlech (Ceramiales), are parasitic on rhodomelacean species belonging to the tribes Chondrieae and Laurencieae. Although Janczewskia and Ululania are classified in the same tribes as their host species, the taxonomic placement of Benzaitenia has been controversial. To infer the phylogenetic positions of these parasites and to clarify the relationships between the parasites and their hosts, phylogenetic analyses of partial nuclear SSU and LSU rRNA genes and the cox1 gene were performed. The SSU rRNA gene analyses clearly show that both Janczewskia species are positioned within the Laurencia s. str. clade with their host species, while Benzaitenia and Ululania are placed in the Chondrieae clade. According to these analyses, J. hawaiiana and U. stellata are not sister to their current hosts; in contrast, B. yenoshimensis and J. morimotoi are closely related to their current hosts. These data suggest that J. hawaiiana and U. stellata have likely evolved from species other than their current hosts and have switched hosts at some point in their evolutionary history. Likelihood ratio tests do not support the monophyly of J. hawaiiana and J. morimotoi, suggesting multiple origins of parasitism within Laurencia s. str.