The Pliocene Ringold Formation of eastern Washington has yielded important new materials of tremarctine bears of the anagenetic Plionarctos lineage. The genus is reviewed in light of this new material and observations made on other described specimens. One of these was previously described from the medial Hemphillian Rattlesnake Formation of Oregon and is recognized as a tremarctine bear, thus extending the earliest record of the group into the early part of the late Miocene. The late Hemphillian P. edensis, the genotypic species, is rediagnosed, although no new material is added to its hypodym. A new species from the early Blancan White Bluffs sites, P. harroldorum, appears to have been derived from P. edensis. Referred Plionarctos sp. from the medial Blancan Taunton Locality has some dental features that are more derived and approach those of the Pleistocene Tremarctos floridanus. Plionarctos forms a paraphyletic stem-group for the Tremarctinae. Species of this genus can be traced successively into the Pliocene where they form the stock from which the Pleistocene and Recent species of Tremarctos, Arctodus, and Pararctotherium arose.