This study combines morphological and molecular data to address several questions of species validity within the dragonfly genus Sympetrum. We compared morphological characters (genitalia and other putatively diagnostic characters) and DNA sequences from mitochondrial cytochrome oxidase I (COI) and nuclear internal transcribed spacer (ITS) regions between these disputed taxa and their close relatives. Specimens of Sympetrum nigrescens Lucas shared COI haplotypes with Sympetrum striolatum (Charpentier), and no morphological characters consistently diagnosed S. nigrescens, which therefore becomes a junior synonym of S. striolatum. Similarly, Sympetrum occidentale Bartenev shared identical COI and ITS sequences with Sympetrum semicinctum (Say), and the supposed diagnostic morphological characters overlapped with the intraspecific variation within S. semicinctum. Sympetrum occidentale becomes a junior synonym of S. semicinctum. In a third case, the genetic distance between Sympetrum signiferum Cannings & Garrison and Sympetrum vicinum (Hagen) was lower than that found between most undisputed species. However, the morphological characters that distinguish S. signiferum from S. vicinum were distinct and consistent, and they supported the retention of S. signiferum as a valid species. In the fourth case, neither morphological nor genetic data were able to distinguish Sympetrum janeae Carle consistently from Sympetrum internum Montgomery, or Sympetrum rubicundulum (Say); in addition, genetic distances between individuals of S. internum and S. rubicundulum were small or nonexistent. Further studies are necessary to test the species status of S. janeae and its close relatives.