Abstract: Siganus oramin is an important economic fish species primarily distributed in the coastal waters of southeast China. We analyzed the genetic diversity of 197 individuals of seven S. oramin populations, including Beibu Bay (BB), Wenchang (WC), Shaba Bay (SB), Dapeng Bay (DP), Daya Bay (DY), Dongshan Bay (DS), and Sansha Bay (SS), based on mitochondrial cytochrome b (Cyt b) gene sequences. The Cyt b gene sequence was 1 141 base pairs in length, containing 140 variable sites and defining 81 haplotypes. The overall haplotype diversity was 0.921±0.014, and the nucleotide diversity was 0.006 4±0.001 0. The genetic distance and differentiation indexes showed no significant genetic differentiation among the seven populations, and both the neighbor-joining tree and haplotype network structure suggest little genetic differentiation. AMOVA analysis reveals that most variation (99.83%) was found within populations. The results of the neutral test show that the Fu's Fs value was significantly negative (−24.605, p<0.05) and the Bayesian skyline plot analysis reveals that the S. oramin population underwent a demographic bottleneck followed by a rapid population expansion over a short evolutionary time span, which suggests that S. oramin might have experienced a bottleneck effect and population expansion within a short period of time, with the estimated expansion time being about 15.77 thousand years ago.