Three new species of Lophomastix Benedict, 1904, L. boykoi, L. kellyi, and L, altoonaensis are described from the upper Eocene-upper Oligocene Lincoln Creek Formation and the lower to middle Miocene Astoria Formation of Washington State, USA. Previously only one other fossil Lophomastix, L. antiqua Schweitzer and Boyko 2000, had been described from the upper Eocene Quimper Formation of Washington. The two extant species of Lophomastix have a limited geographic distribution in the northwestern Pacific Ocean from Russia to Korea and in the northeastern Pacific from southern California to Baja California (Boyko, 2002). The description of two new Eocene and one new Miocene species suggests Lophomastix originated and was well established in the northeastern Pacific from the late Eocene through at least the early to middle Miocene.