Species of the red algal genus Acrochaetium are present in the global marine flora, but at best only preliminary assessments of species diversity and phylogeny have been attempted for the genus. During an ongoing floristic survey of Canadian seaweeds, we routinely encountered species assigned to this genus epibiotic on other organisms, which provided the material and motivation for the current study. Seven Atlantic, three Pacific and a single subarctic species were identified in analysis of COI-5P (cytochrome oxidase subunit I) sequences with typical levels of intraspecific (0–0.3%) and interspecific (7.9–19.3%) variation observed except between western and eastern Atlantic populations of A. humile (0.9%). Within a historical Atlantic species ‘complex’, we established that A. virgatulum is synonymous with A. secundatum; however, A. luxurians was an independent lineage and is resurrected. A Pacific isolate of A. secundatum was collected and represents an extension of the reported range for this species. In a second complex, Pacific A. arcuatum and A. vagum were synonymous, as suggested in the literature previously, but A. porphyrae was genetically and anatomically distinct. Further, A. porphyrae did not group with Acrochaetium in nuclear-encoded LSU rRNA and combined LSU/COI-5P analyses, and Rhododrewia gen. nov. is proposed for R. porphyrae (Drew) comb. nov.