Eight species of the bat genus Eptesicus are distributed in South America of which four are known from Argentina. Here, we describe a new species, based on specimens collected from La Rioja Province, and one specimen from Mendoza Province, Argentina, deposited in a museum collection. This new species of Eptesicus can be distinguished from other congeners by its golden to yellowish-brown coloration, strong contrast between dorsal and ventral fur color, long forearm (> 41 mm), and by the relatively short (∼ 6 mm) dorsal fur. A detailed description of the external and cranial morphology of the new species, as well as comparisons with other species of similar size, and present in the same distributional area, are included. A Wilcoxon–Mann–Whitney test and a discriminant linear function analysis were performed to assess the morphometric differences among species of Eptesicus. The phylogenies obtained by different criteria, as well as those obtained independently from different genes, are in agreement with regard to the relationships of the new species.