Protopselaphinae is a monogeneric subfamily of Staphylinidae comprising eight extant and one extinct species of minute rove beetles. We describe a tenth species of this enigmatic subfamily, Protopselaphus newtoni sp. nov., from mid-Cretaceous Burmese amber (similar to 99 Ma). Protopselaphus newtoni sp. nov. differs from its congeners in its smaller body size and antennal structure. Uniquely, the apical maxillary palpomere of the new species is longer than maxillary palpomere 3. A key to the extant and fossil members of the genus Protopselaphus is provided. Extinct species of the subfamily Protopselaphinae are smaller than extant species, a trend also apparent in some other beetle lineages present in Burmese amber, opening new questions about the evolution of insect body size in the Mesozoic.