A new uncommon epilithic diatom, Eunotia relicta sp. nov., is described from a small oligotrophic, acidic stream in the Chapada Diamantina region, northeast Brazil. Eunotia relicta is a typical eunotioid form with a pronounced arch on the dorsal side of the valve mainly in smaller valves. Contrary to most Eunotia species, Eunotia relicta does not possess rectangular frustules in girdle view. Until now, this characteristic was only observed in a few species such as E. charliereimeri Edlund & Brant, E. sarraceniae E.E. Gaiser & J.R. Johansen and E. arcuoides Foged, all of them considered asymmetric along the valve plane. Some interesting morphological features of the new species are broad copulae on the dorsal side, an inconspicuous single conical spine on the valve face near the margin at both poles, and the irregularly thickened interstriae. Moreover, each valve has irregular projections at the dorsal valve face margin/mantle juncture and up to three rimoportulae per valve located on the mantle. These features associated with the distinct valve outline set it apart from E. charliereimeri, E. sarraceniae and E. arcuoides, which have no spines, smooth valve faces and 1–2 rimoportulae per valve.