The taxonomy of the small Neotropical Molossus species has been notoriously difficult due to a lack of adequate comparative material. One taxon in particular, Molossus barnesi Thomas, 1905 was believed to be restricted to a narrow stretch of coastal areas around Cayenne, in French Guiana and was so far represented only by three female specimens. It was variously considered as a species on its own, or synonymized with Molossus molossus or Molossus coibensis. Thanks to the discovery of several mixed colonies of these small molossids in two localities in French Guiana, we could obtain and measure a large sample (nearly 200 specimens) of adult individuals to better assess their morphological variation. Owing to largely bimodal and non-overlapping distributions of external measurements such as forearm length, we could demonstrate the existence of two sympatric morphotypes, the smaller one corresponding to M. barnesi and the larger one to M. molossus. Univariate and multivariate comparisons of cranio-dental and external characters further suggest that the new series of barnesi from French Guiana do not differ notably from specimens assigned to M. coibensis from elsewhere. Molecular reconstruction based on the barcode gene (CO1) confirmed their genetic distinctness, but also the overall close relationships (mean divergence of 1.7%) of all assayed taxa in this group. Although none of the haplotypes are shared across taxa, haplotypes of M. coibensis from Panama and M. barnesi from French Guiana are mixed in a single, poorly supported cluster, suggesting that these animals could represent a single biological species. Based on all evidences, we thus recommend treating barnesi as a junior synonym of M. coibensis, a species now widely and continuously distributed from Central America to Middle South America.