The relationship between ozone (O₃) exposure and gestational diabetes mellitus (GDM) in twin pregnancies remains unexplored. This study aimed to investigate the association between O₃ exposure and GDM risk in twin pregnancies, and to explore the synergistic effects of O₃ exposure with other maternal factors. A total of 428 pregnancies recruited from a prospective twin cohort were included. Cox proportional hazard models with distributed lag non-linear models (DLNMs) were applied to examine the associations between O₃ exposure and the risk of GDM and to identify the critical windows. The multiplicative and additive interaction were further analyzed to test the synergistic effects. A 10 μg/m³ increase in average O₃ exposure during the 12 weeks before pregnancy was associated with a 26% higher risk of GDM. The critical windows were identified in the period from the 3rd week before gestation to the 2nd gestational week as well as from the 17th to 19th gestational week. There were synergistic effects between high O₃ exposure during preconception and advanced maternal age, and a history of preterm birth/abortion/stillbirth. Periconceptional O₃ exposure could increase the risk of GDM in twin pregnancy women, and the synergism of O₃ exposure with certain GDM risk factors was observed.