Background and Objectives: The prevalence of gestational diabetes mellitus (GDM) significantly varies across different ethnic groups. In particular, Africans, Latinos, Asians and Pacific Islanders are the ethnic groups with the highest risk of GDM. The aim of this study was to evaluate the impact of ethnicity on pregnancy outcomes in GDM. Patients and Methods: n = 399 patients with GDM were enrolled, n = 76 patients of high-risk ethnicity (HR-GDM), and n = 323 of low-risk ethnicity (LR-GDM). Clinical and biochemical parameters were collected during pregnancy until delivery. Fetal and maternal short-term outcomes were evaluated. Results: HR-GDM had significantly higher values of glycosylated hemoglobin checked at 26–29 weeks of gestation (p < 0.001). Gestational age at delivery was significantly lower in HR-GDM (p = 0.03). The prevalence of impaired fetal growth was significantly higher in HR-GDM than LR-GDM (p = 0.009). In logistic regression analysis, the likelihood of impaired fetal growth was seven times higher in HR-GDM than in LR-GDM, after adjustment for pre-pregnancy BMI and gestational weight gain (OR = 7.1 [2.0–25.7] 95% CI, p = 0.003). Conclusions: HR-GDM had worse pregnancy outcomes compared with LR-GDM. An ethnicity-tailored clinical approach might be effective in reducing adverse outcomes in GDM.