The orthopteran family Mogoplistidae is a rather diverse extant family, however, a rare fossil record has been reported, especially from the Mesozoic. Protomogoplistes inflatus sp. nov., the first known Mesozoic adult of Mogoplistidae, is described based on a well-preserved specimen from mid-Cretaceous Kachin amber in northern Myanmar. It is attributed to the genus Protomogoplistes (Mogoplistidae: Protomogoplistinae). The new taxon is mainly characterized by a long upper inner metatibial spur almost half the length of the basitarsus, and hind basitarsus with dorsal spines and two apical spurs. As the oldest record of adult Mogoplistidae, the new species provides new insights into Mesozoic Mogoplistidae, including the lack of scales, the pronotum not covering the forewing on a long area, the presence of a tympana on the protibia, and a developed mirror and harp on the forewing.