Objective: The aim of this retrospective study was to establish the association between gestational diabetes mellitus (GDM) with the adverse outcomes of twin pregnancies, as well as indentifying the risk factors which contribute to developing GDM in twin pregnancies. ----- Methods: This study included 297 twin pregnancies born at the Clinic for Women's Diseases and Obstetrics of the Clinical Hospital Center Zagreb in the period from January 1, 2019 to December 31, 2021. The study excluded all twin pregnancies aborted / born before 28th week of gestation, women who did not undergo an oGTT test and mothers of twins with pre-existing diabetes mellitus. The diagnosis of gestational diabetes (GDM) was made on the basis of one or more pathological values of the oGTT test. Depending on the type of twin pregnancy and the presence of GDM, the subjects were divided into four groups: BC GDM +, BC GDM-, MC GDM +, MC GDM-. Perinatal and early neonatal outcome were analyzed across all four groups. ----- Results: Our results show the association of BMI before pregnancy and family history of DM as risk factors for the development of GDM in pregnant women with MC and DC twin pregnancies (β=0.0403, R=0.20, p=0.0481). The largest proportion of women who conceived through IVF was in the DC pregnancy group (p<0.001), but IVF was not presented as a risk factor for the development of GDM. By looking at the incidence of perinatal and neonatal complications, we concluded that women pregnant with DC or MC twins complicated by GDM underwent labour earlier (p=0.039), which is consistent with the more frequent occurrence of neonatal complications (p=0.010) in those groups. Based on the results of this study, MC and DC twin pregnancies and GDM are positively correlated with the occurrence of perinatal and neonatal complications. For example, the correlation of the latter with intrauterine death of one twin and discordant growth (R=0.36, p=0.0039) was shown in terms of perinatal complications, as well as complications of prematurity of both twins, neonatal death and infection of both twins in the form of neonatal complications (R=0.33, p =0.0433). ----- Conclusion:. The results of this study indicate a statistically higher frequency of perinatal and early neonatal complications in monochorionic and dichorionic pregnancies with GDM compared to twin pregnancies without GDM. Quality antenatal care aimed at twin pregnancies and proper glycemic control during pregnancy are important for reducing perinatal complications and giving birth to healthy newborns.