Acanthosis nigricans (AN) usually correlates to insulin resistance (IR) or obesity in obese populations, but adequate studies on the significance of AN in people with normal body mass index (BMI) have not been performed and discussed. Three hundred and thirty-nine polycystic ovary syndrome (PCOS) patients with normal BMI (<23 kg/m2) were recruited. The anthropometric and biochemical parameters of these patients were measured. In these patients with normal BMI, 33 (9.7%) women had AN, and six (1.77%) women were diagnosed with metabolic syndrome. Most of the anthropometric and biochemical variables associated with metabolic status were more unfavorable in the AN-positive group compared with the AN-negative groups. The prevalence of central obesity, IR and reduced high-density lipoprotein cholesterol (HDL-C) level were also significantly higher in the AN-positive group (P < 0.05). In multiple regression analysis, presence of AN was still significantly associated with IR (odds ratio [OR] = 2.952, 95% confidence intervals [CI] = 1.367–6.376] and reduced HDL-C level (OR = 2.668, 95% CI = 1.160–6.135) after adjustments for age and BMI. Sensitivity, specificity, and positive and negative predictive values for AN to detect IR were 18.6%, 92.6%, 39.4% and 81.4%, respectively. In conclusion, presence of AN correlated with IR and reduced HDL-C level in PCOS women with normal BMI. AN status had high specificity to detect IR, but lack of sensitivity.