Premise of research. Platanus leaves are common and readily recognized in Miocene floras across western North America, but different philosophies have been used to distinguish species from these fossil occurrences, and their relationships to modern Platanus have remained uncertain.Methodology. Fossils were studied under reflected light, transmission and epifluorescence microscopy, and microcomputed tomography (μ-CT). Morphological characters used to distinguish extant species of Platanus were applied to collections of Miocene Platanus from California, Oregon, Washington, and Idaho.Pivotal results. We provide an emended diagnosis for P. dissecta Lesquereux with new characters from stipule morphology and epidermal anatomy, and we review the distribution of this species among many localities in the Miocene of western North America. We designate a lectotype for P. dissecta and place P. bendirei (Lesquereux) Wolfe and other subsequently established names in synonymy. Fossil inflorescences, infructescences, and dispersed achenes found associated with P. dissecta are described as P. emryi sp. n.Conclusions. We determine that P. dissecta conforms to Platanus subg. Platanus in its enlarged petiole bases and lobed laminae but differs from modern Platanus species in that its associated capitula are borne on thicker axes, which we infer to have been longer than those of extant species. We find that P. dissecta bears close similarity to P. orientalis, P. racemosa, and P. gentryi and infer that it may have belonged to the Pacific North American/European clade within Platanus subg. Platanus that contains these three extant species and is readily distinguished from the eastern North American/Central American clade containing P. occidentalis and P. mexicana.