A leptochromine ant-like stone beetle, †Rovnoleptochromus balticussp. nov., is described, based on an exquisitely preserved inclusion in Upper Eocene Baltic amber from Kaliningrad Region. The holotype specimen reveals novel characters, not observed (presumably because of poorer preservation) in the previously known †R. ableptonoides Jałoszyński & Perkovsky of Rovno, Ukraine: three pairs of thick, long setae on the head (on vertex, near each eye and near each antennal insertion). Similar, presumably sensory structures (sensilla chaetica), are known in extant and extinct (Upper Cretaceous) Leptomastacini and the enigmatic Upper Cretaceous †Palaeoleptochromus O'Keefe. However, in these taxa, at most two pairs occur. The new finding broadens the known distribution of †Rovnoleptochromus during the Eocene, and suggests that the paired sensory bristles on the head may represent an ancestral condition in at least some of the tribes currently included in Mastigitae, which have later been partly or completely reduced in most members of this group.