Summary: Halarachnion is a newly reported genus for the Gulf of Mexico where it is represented by an undescribed species growing attached to rhodoliths at 52-73m depth offshore Louisiana in the northwestern Gulf, USA. Aspects of the vegetative and female reproductive morphology of the Gulf species are documented, and compared to of Halarachnion latissimun from Japan. Halarachnion louisianensis sp. nov. is a monoecious, presumably heteromorphic species as inferred from the absence of tetrasporophytic specimens in our collections. Halarachnion latissimum, in contrast, is a dioecious species that is isomorphic. A distinct reproductive character that differentiates both species is the behavior of a connecting filament of which a lateral process fuses with an auxiliary cell lobe and effects more than one diploidization in H.louisianensis versus directly fusing with an auxiliary cell and effecting only a single diploidization in H.latissimum. Comparative chloroplast-encoded rbcL sequence analysis indicates that the closest species relationship of isolates from the Gulf of Mexico remains equivocal, whereas reproductive morphology points to a closer affinity with H.ligulatum. This is the first report of the Furcellariaceae, a family in the Solieriaceae-complex, for the Gulf of Mexico.