Based on morphological evidence, the systematic position of the genus Petrospongium and its relationship with the genera Leathesia and Corynophlaea have long been disputed. In the present study, the position within the order Ectocarpales of the type species, Petrospongium berkeleyi (Grev. in Berk.) Nägeli ex Kütz., has been investigated using molecular phylogenetic analysis based on rbcL and psaA sequences. Petrospongium berkeleyi and its Pacific vicariant P. rugosum appeared closely related and did not cluster with other members of the Chordariaceae, particulalrly the genera Leathesia and Corynophlaea, nor with any other currently recognised ectocarpalean family. The genus Petrospongium formed an independent monophyletic group which was a sister clade of the family Ectocarpaceae. The genera Ectocarpus and Petrospongium differ in their mode of thallus construction (filamentous thallus and pseudoparenchymatous cushion-like thallus, respectively), and in the type of plastids (ribbon-shape and discoid, respectively). These morphological differences agree with our phylogenetic analyses and lead us to propose the family Petrospongiaceae fam. nov. to accommodate the genus Petrospongium.