Fossil hominin mesosterna, while scarce, can provide useful morphological data in addition to rib remains regarding aspects of thoracic size and shape. These data, in turn, can address hypotheses related to respiratory dynamics, climatic adaptation, and ecogeographical patterning. In this study, we re-evaluate the anatomical representation of the mesosternum of the Regourdou 1 Neandertal individual that alters key aspects of the original description of the fossil remains. We compare this specimen together with the mesosterna of the Kebara 2 Neandertal male individual and the Tabun C1 Neandertal female individual to a large extant modern sample. Our study shows that the current evidence available for Neandertals indicates longer mesosterna, reflecting larger thorax sizes among Neandertals, in comparison with extant humans. Additionally, while this study weakens previous suggestions of ecogeographically mediated differences in the size and shape of upper thorax between Neandertals from the Mediterranean Levant and those deriving from Western Europe, we cannot unambiguously disprove the notion of such clinal differences.