Small subunit (SSU) rDNA was sequenced for 25 species in 19 genera of the Gigartinales (Rhodophyta). As well, the internal transcribed spacer (ITS) region was sequenced, and a data matrix of 36 morphological characters was constructed for 16 species of Dumontiaceae. Phylogenetic trees were calculated from a multiple alignment of the SSU sequence data to infer relationships between species of Dumontiaceae and other gigartinalean taxa. The SSU analysis produced a polyphyletic Dumontiaceae. Notably, Acrosymphyton failed to associate with the included Gigartinales, let alone the Dumontiaceae, supporting an earlier proposal to remove it to a new family. The analyses were equivocal about the phylogenetic affinities of Dudresnaya, which clustered with the Kallymeniaceae, and the affinities of the Indo‐West Pacific Gibsmithia, Kraftia, and Dasyphloea, the last‐mentioned clustering with the Antarctic Gainiaceae, and these four taxa with Portieria (Rhizophyllidaceae). Further investigations are necessary to resolve relationships among these taxa. Rhodopeltis, a genus recently moved to the Dumontiaceae from the Polyideaceae, showed a weak association with the remaining northern Dumontiaceae. The final group consisted of cold‐temperate Northern Hemisphere species. Phylogenetic analyses using a combination of SSU, ITS, and morphological data within this clade produced two strongly supported clades, a Dilsea/Neodilsea clade and a Cryptosiphonia/Dumontia clade. Dilsea is derived from a paraphyletic Neodilsea and may itself be polyphyletic. Atlantic and Pacific isolates of Dumontia contorta clearly showed sufficient divergence to warrant recognition as distinct species, and Dumontia alaskana, sp. nov. is proposed for the Pacific species.