Objectives We describe a hominin permanent lower left third premolar unearthed in 1997 at Walou Cave (Belgium), found in direct association with a Mousterian lithic industry, in a layer directly dated to 40–38,000 years BP. Materials and methods The taxonomical attribution of the tooth is addressed through comparative morphometric analyses, and stable isotope analyses aimed at determining the diet of the individual. Results The Walou P3 plots within the Neandertal range of variation and is significantly different from recent modern humans in all morphometric assessments. The isotope data showed that like other Neandertals, the Walou individual acquired its dietary proteins primarily from terrestrial food sources. Discussion We discuss the implications of the existence of a clearly Neandertal premolar dating to the period of the Middle to Upper Paleolithic transition in the Meuse river basin.