Species-delineation studies over wide geographic ranges often reveal diverging conclusions that may ultimately improve our understanding of biogeographic and evolutionary processes. In this study, we investigated both traditional morphological characteristics and genetic variances between worldwide populations of the nine pelvic-fin ray marine sardine species of the subgenus Sardinella (Clupeidae). Our detailed traditional taxonomic investigations, including geometric morphometric analyses have firmly reduced the currently valid five species into two significantly different morphotypes: the short head Sardinella aurita and the long head S. longiceps, based mainly on head-length proportions and number of lower gillrakers. However, genetic analyses, using three mitochondrial genes and one nuclear gene, have revealed an exceptional low genetic divergence within the investigated species, regardless morphotype or geographic distances. Maximal average differences between the sampled localities were 0.6%, 1.8%, 1.8%, and 2.2% in 16S, COI, Cytb and S7 genes, respectively. These ranges of genetic differences better suit an average intra-specific variation within distant populations of marine fishes, implying the possible presence of a single valid species with an exceptional ecophenotypic variation, thus entitling the senior species S. aurita as the world's most widespread clupeid. Investigating the variation between the populations has revealed expected geographic sub-structuring, with few exceptional cross-regional shared haplotypes. Considering its utmost importance to worldwide fisheries, this finding of remarkable genetic homogeneity should draw future debate among population geneticists and fishery researchers.