Fossil capitate hydrozoans require exceptional conditions for preservation. Here we describe Bertratis ciurcae new genus, new species from the Silurian (Pridoli) of southern Ontario and upper New York State, where it occurs in association with a diverse assemblage of eurypterids. Only the float (pneumatophore) is well preserved, surviving as a thick carbonaceous compression. The new taxon is the largest fossil capitate reported, reaching a width of 17 cm, and the third Porpita-like example known from the Paleozoic. It was a rare pelagic component of the biota of the well-known Bertie Group Lagerstätten.