A revision of the hydrophilid subfamily Helophorinae (Coleoptera), containing the single genus Helophorus Fabricius 1775, is presented, including an historical review, a discussion of diagnostic characters and techniques, and an attempt at a phylogenetic reconstruction of the major lineages of the Nearctic Helophorus.Forty-one species are recognized; 9 species and 1 subspecies are described as new.Atractohelophorus Kuweit 1886 is placed in synonymy with Rhopalelophorus Kuwert 1886, and Megahelophorus Kuwert 1886 with Helophorus Fabricius 1775. On the specific level H. ventralis Motschulsky 1860 and H. obsoletesulcatus Motschulsky 1860 are placed in synonymy with H. lineatus Say 1823.A neotype is designated for H. lineatus Say.Lectotypes are designated for H. oblongus LeC., H. fortis LeC., H. regularis Sharp, H. lacustris LeC., and H. alternatus LeC.The genus Helophorus is described and the nomenclatorial and taxpnomic problems surrounding it are discussed in detail. Keys to subgenera and species are provided. Each subgenus is described; each species is described and illustrated, all available bionomic and distributional data are presented, and distributional records are mapped.