Background: Gestational weight gain (GWG) is a key determinant of maternal and fetal outcomes, particularly in pregnancies complicated by diabetes. Both excessive and inadequate GWG have been associated with adverse outcomes, yet data on the impact of GWG in diabetic mothers in Bangladesh remain limited. This study aimed to evaluate the association between GWG and maternal and fetal complications in women with gestational diabetes mellitus (GDM) and pregestational diabetes mellitus (PGDM). Methods: A cross-sectional analytical study was conducted at the Department of Obstetrics and Gynecology, BIRDEM General Hospital, Dhaka, from August 2022 to June 2024. A total of 102 pregnant women at ≥37 weeks of gestation, including 51 with GDM and 51 with PGDM, were enrolled. Participants were categorized into excessive, normal, and inadequate GWG groups. Maternal and fetal complications, as well as mode of delivery, were analyzed using appropriate statistical tests. Results: Among GDM patients, 49.02% had normal GWG, 31.37% excessive, and 19.61% inadequate. Cesarean section was significantly higher in the excessive GWG group (93.7%) compared to normal (40.0%) and inadequate (60.0%) GWG groups (p=0.006). Maternal complications were more frequent in PGDM than GDM across all GWG categories but without statistical significance. Fetal complications were significantly higher in PGDM mothers with normal GWG (57.9% vs. 28.0%, p=0.046), while other GWG groups showed no significant differences. Type-specific fetal complications in inadequate GWG were not statistically different between GDM and PGDM. Conclusion: Both excessive and inadequate GWG in diabetic pregnancies are associated with increased maternal and fetal complications, with PGDM mothers at higher risk. Appropriate monitoring and management of GWG is crucial to optimize perinatal outcomes.