Objective To investigate the impact of maternal prepregnancy body mass index (BMI) on maternal and infant outcomes in twin pregnancies. Methods A retrospective analysis included 4 824 twin pairs and their mothers with complete records, delivered at 21 tertiary grade-A hospitals nationwide from January 2018 to December 2020. Participants were grouped by prepregnancy BMI: underweight (<18.5 kg/m²; n=566), normal weight (18.5 to <24.0 kg/m²; n=3 120), overweight (24.0 to <28.0 kg/m²; n=892), and obese (≥28.0 kg/m²; n=246). Differences and associations in adverse maternal and infant outcomes were compared among groups. Results The gestational age in the overweight group was lower than in the normal and underweight groups, with a higher rate of very preterm birth (P<0.008). Compared with the normal and overweight groups, the proportion of small-for-gestational-age (SGA) neonates was higher in the underweight group (P<0.008). After adjustment for confounders, the obese group had higher risks of gestational diabetes mellitus and hypertensive disorders of pregnancy than the normal and underweight groups (all P<0.001). Compared with the normal group, the underweight group had a higher risk of SGA among twins (P<0.05). Conclusions Maternal prepregnancy BMI is closely associated with maternal and infant outcomes in twin pregnancies, providing evidence-based support for individualized weight management.