Lomatium tamanitchii (Apiaceae), is a newly discovered substrate-specific, narrow endemic species. The species grows on clay soils in grassland swales and gentle slopes in the Columbia Hills of eastern Klickitat County in south-central Washington State. A small disjunct occurrence has also been recently recognized in Union County, Oregon. The species is most typically distinguished by a multi-branched caudex surmounting a large, thick, blunt-tipped taproot. It is identified by its sparsely to densely short hairy leaves with broadly winged petioles, and its narrowly elliptical dorso-ventrally compressed fruits that have short hairs and distinct narrow raised ventral ribs. Lomatium tamanitchii is clearly distinct from all other species in the genus as based primarily upon gaps in characters of fruit morphology and vestiture. Populations occur in dense near-monocultures strictly confined to shrink-swell soils derived from devitrified silicic volcanic ash on massive landslide deposits. The known range of L. tamanitchii is restricted to a small area in eastern Klickitat Co., Washington and several hundred plants in a newly-discovered disjunt population approximately 180km distant in Union Co., Oregon. This limited distribution raises conservation concerns. Lomatium tamanitchii is compared with morphologically similar taxa growing in nearby areas of the Columbia Basin.