The specific effects of gestational diabetes mellitus (GDM) on twin pregnancy outcomes, which are at high risk per se, are unclear. The present study analyzes outcomes of twin pregnancies complicated by GDM (n = 227) by comparing them with GDM singleton pregnancies (n = 1060) and with twin pregnancies without GDM (n = 1008), all followed up at Sant’Anna Hospital, Turin (Italy), between January 2010 and March 2020. The prevalence of GDM among twin pregnancies (n = 1235) was 18.4%. Compared to GDM singletons, GDM twins had higher rates of preeclampsia (aOR 2.0; 95% CI 1.2–3.8), cesarean section (aOR 7.5; 95% CI 5.2–10.8), and neonatal hypoglycemia (aOR 2.5; 95% CI 1.1–5.3). They had a higher incidence of abnormal 2 h OGTT values (aOR 7.1; 95% CI: 3.2–15.7) and were less likely to require insulin therapy (aOR 0.5; 95% CI: 0.3–0.7). In comparison with twin pregnancies without GDM, women with GDM twins were significantly older (35.0 vs. 33.0 years;p< 0.001) and had higher BMI (23.0 versus 22.0 kg/m2;p< 0.001); they had a higher incidence of LGA newborns (aOR 5.3; 95% CI 1.7–14.8), and lower incidence of low APGAR scores (0.5; 95% CI 0.3–0.9). Overall, GDM does not worsen outcomes of twin pregnancy, which is per se at high risk for adverse outcomes.