Liparis adiastolus, a previously undescribed species from the coasts of Oregon and Washington, northwest United States, is described. The new species has probably been consistently confused with Liparis rutteri, which has a more northerly distribution. Liparis adiastolus is clearly distinguishable from all similar species in having 41–70 pyloric caeca, 35–39 vertebrae, 30–33 dorsal-fin rays, 32–35 pectoral-fin rays, disk 67–77% HL, and other distinctive characters. In addition to describing the new species, we select a lectotype of L. rutteri, provide an expanded diagnosis for that species, and discuss the confusion previously existing with regard to its identification.