OBJECTIVE Gestational diabetes and obesity are independent risk factors for obstetric and neonatal complications. The purpose of our study was to evaluate the impact of their association on pregnancy outcomes. METHODS Monocentric retrospective cohort study including patients with obesity and gestational diabetes (GDM), those with GD without obesity, and those with obesity without GDM, who gave birth between 01 January 2012 and 31 December 2014, and whose GDM was exclusively monitored at our centre. The diagnostic criteria and management modalities of GDM were based on the 2010 CNGOF (Collège National des Gynécologues et Obstétriciens Français [French national college of obstetricians and gynaecologists]) Clinical Practice Recommendations. Obesity was defined as having a body mass index ≥30 Kg/m2. RESULTS A total of 1,484 patients were included, 259 with GDM and obesity, 549 with GDM without obesity, 676 with obesity without GDM. In the GDM + obesity group, GDM was treated earlier and was more uncontrolled and more often treated with insulin in relation to non-obese women with GDM. These patients also presented a higher risk of caesarean section (OR 2.92, CI 95% 2.04-4.16, P<0.001), preeclampsia (OR 4.62, CI 95% 1.31-16.32, P=0.017), maternal morbidity (OR 2.05, CI 95% 1.37-3.04, P<0.001) and large foetus for gestational age (OR 1.91, CI 95% 1.26-2.88, P=0.002). Obesity alone was a risk factor in its own right for preeclampsia (OR 7.32, CI 95% 2.50-21.45, P<0.001) and macrosomia (OR 3.55, IC 95% 2.24-5.62, P<0.001), compared to non-obese patients with GDM. Uncontrolled GDM was associated independently of obesity with the risk of induced labour and large foetus for gestational age. CONCLUSIONS Obesity on its own is a risk factor for obstetric complications and its association with GDM strongly impacts on pregnancy outcomes.