A new species of a staphylinid beetle, Paleothius mckayi sp. n. is described using a single fossil from the Orapa Diamond Mine in Botswana. Paleothius is placed in the extant subfamily Staphylininae based on its general symmetrical habitus, typical sub-limuloid and elongated medium-to-large body form, elongated head, remarkably long antennomere 1, antennal density increasing apically, antennal insertions closer to the eye than to each other, tapering abdomen with two pairs of paratergites, and intersegmental membranes with ‘brick-wall’ pattern. It is separated from its sister clade Paederinae by having exposed antennal insertions and a small post-coxal process. It can be distinguished from all other Staphylininae by its significantly curved temples and large subtriangular pro-, meta- and meso-coxae. The specimen is preserved with flexed, distally-sharpened, extended or protruding scissor-like mouthparts. It is possible that it was primarily using these extended or protruding scissor-like mouthparts to actively hunt prey in leaf litter and other microhabitats in and around the crater lake at Orapa. This fossil further confirms the widespread distribution of staphylinines during the Cretaceous. Moreover, similarly to other previously described fossil staphylinine staphylinid beetles, it portrays morphological stasis, or morphological conservatism. This is also known as arrested evolution. Therefore, it provides evidence for punctuated evolution dating back to the Cretaceous. Moreover, it highlights that morphological similarities between Staphylininae and Paederine date back to the Jurassic period.