Extant baleen whales (Mysticeti) use the characteristic keratinous baleen for filter-feeding without functional dentition, but the fossil record clearly shows that “toothed” baleen whales have been around since the late Eocene.1 Globally, only two Eocene mysticetes have been found, and both are from the Southern Hemisphere: Mystacodon selenensis from Peru, 36.4 Ma1,2 and Llanocetus denticrenatus from Antarctica, 34.2 Ma.3,4 Based on a partial skull from the lower part of the Lincoln Creek Formation in Washington State, USA, we describe the geochronologically earliest mysticete, Fucaia humilis sp. nov., in the Northern Hemisphere. Our fieldwork and careful geological age control places Fucaia humilis sp. nov. in the latest Eocene (ca. 34 Ma, near the Eocene/Oligocene transition at 33.9 Ma), approximately coeval with the oldest record of fossil kelps.5,6 This observation leads to our hypothesis that the origin and development of a relatively stable, nutrient-rich kelp ecosystem5,6 in the latest Eocene may have fostered the radiation of small-sized aetiocetids in the North Pacific basin, a stark contrast to the larger llanocetids with the onset of the Antarctic Circumpolar Current in the latest Eocene in the Southern Hemisphere.7-9 Our discovery then suggests that disparate mechanisms and ecological scenarios may have nurtured contrasting early mysticete evolutionary histories between two hemispheres.