A species previously confused with Ahnfeltiopsis devoniensis in Ireland and with Gymnogongrus crenulatus in eastern Canada and north-eastern USA has remained undescribed because of its cryptogenic origin. Our published research suggested a trans-Atlantic introduction, possibly with ship ballast rock, but the relationship with A. leptophylla from California required further investigation and a North Pacific origin was possible. Here, we report that 34 samples of this species from Northern Ireland (UK), New Hampshire (USA) and New Brunswick (Canada) were genetically identical at the cox2-3 spacer locus, consistent with a recent introduction. By contrast, in A. leptophylla, four haplotypes were found in 9 samples from three sites in California. This species (as G. crenulatus) was recently discovered in the North Pacific for the first time, during surveys in British Columbia for the Canadian Barcode of Life Project. Phylogenetic analysis of plastid-encoded rbcL and mitochondrial cox1 gene sequences separate it from A. leptophylla in a robust clade with two North Pacific taxa, Gymnogongrus chiton (M.A. Howe) P.C. Silva et De Cew, and "Ahnfeltiopsis gigartinoides", both from California. It is here described as a new species in a new genus, Fredericqia deveauniensis, gen. et sp. nov. The two other members of the genus are Fredericqia chiton (M.A. Howe) comb. nov. and Fredericqia decewii, sp. nov., previously known as Ahnfeltiopsis gigartinoides but not conspecific with the type from Mexico. Fredericqia species form extensive basal holdfasts giving rise to terete or compressed erect gametophytic axes 50-300 mm high. Procarps develop either into compact cystocarps with multiple ostioles but no mucilaginous sheath, or externally protruding carpotetrasporophytes (in F. chiton). Life histories are potentially heteromorphic (erect gametophytes and crustose sporophytes with compact, coalescent hypobasal tissue, forming catenate sporangia, both reproduce apomictically in F. deveauniensis), or carpotetrasporangial. F. deveauniensis differs from the other members of the genus in forming internal cystocarps on flattened blades.