Plionarctos is an extinct genus of short-faced bear endemic to North America from the Late Miocene to the Pliocene.

Described by Childs Frick in 1926, Plionarctos is the oldest known genus within the subfamily of the short-faced bears (Tremarctinae), and is believed to be ancestral to Arctodus, Arctotherium and Tremarctos. Once suggested to have emerged from the Ailuropodinae bears (Agriotherium and Indarctos), Plionarctos probably evolved from Ursavus which emigrated into North America from Eurasia during the Miocene, or possibly Protarctos. A new species, Plionarctos harroldum, was described in 2001 from the White Bluffs Fauna in Washington from remains previously attributed to Protarctos abstrusus. Middle Pleistocene species from France were described (Plionarctos stehlini and Plionarctos telonensis) but have since been reassigned & synonymized with the Asian black bear (Ursus thibetanus).

Various dental characteristics link Plionarctos with other tremarctine bears, such as a deep mandible underneath the molars, a notable groove underneath the premolars, a masseteric fossa at its deepest in line with the teeth, an M1 molar with a labial enamel bulge and high talonid with a very shallow basin, and a rectangular M2 molar.

Archaic characteristics which separate Plionarctos from other tremarctines such as Arctodus and Tremarctos include a lack of a premasseteric fossa (and the associated extension of the strongly ridged masseteric crest) and a tri-rooted (rather than birooted) P4 premolar, with its protocone anterior (instead of behind) the carnassial notch.

In addition to the lack of premasseteric fossa in Plionarctos, the M2 molar of Plionarctos is slightly shorter than that of Tremarctos (though tooth sizes are similar).

Plionarctos harroldorum shares dental characteristics with Tremarctos, with both lacking a well-defined lingual cinguli, and a longer and wider talon on the M2, which is not as dorsal as Plionarctos edensis. The M2 of P. harroldorum is broad compared to the width of M1 with a longer talonid than in P. edensis.

Likely part of the Hemphillian 2 migration wave (Hh2) across the Beringinian land bridge in the late early Miocene, Plionarctos first appeared in North America (during the late Miocene epoch) being recorded from the Rattlesnake Formation of Oregon circa 7Ma. The Hh2 migration was the largest migration wave of Carnivorans between Asia into North America, and was also one of the largest mammalian dispersal events of the Neogene. The ascendancy of Asian fauna in this exchange may have been due to the intensification of the monsoon season in northeastern Asia expanding humid forests into the Arctic (in contrast to the global trend of aridification and expansion of C4 grasslands), with the more static & arid ecologies of North America potentially acting as refugia for Asian fauna.

Due to the ecological diversity of the fauna, the Beringian corridor during the Hh2 migration event is believed to have been a mix of grasslands and forests. Bears were the largest faunal group, with over five species appearing in North America (representing almost all known genera of the period), with their omnivory, locomotive benefits due to large size and overall adaptability likely contributing to their success. Notable fauna within the Hh2 wave includes Plionarctos, Amphimachairodus, Enhydritherium, Indarctos, and Simocyon.