The high overall plant-based diet index (PDI) is considered to protect against type 2 diabetes in the general population. However, whether the PDI affects gestational diabetes mellitus (GDM) risk among pregnant women is still unclear. We evaluated the association between PDI and GDM risk based on a Chinese large prospective cohort – the Tongji Maternal and Child Health Cohort. Dietary data were collected at 13–28 weeks of pregnancy by a validated semi-quantitative FFQ. The PDI was obtained by assigning plant food groups positive scores while assigning animal food groups reverse scores. GDM was diagnosed by a 75 g 2-h oral glucose tolerance test at 24–28 weeks of gestation. Logistic regression models were fitted to estimate OR of GDM, with associated 95 % CI, comparing women in different PDI quartiles. Among the total 2099 participants, 169 (8·1 %) were diagnosed with GDM. The PDI ranged from 21·0 to 52·0 with a median of 36·0 (interquartile range (IQR) 33·0–39·0). After adjusting for social-demographic characteristics and lifestyle factors etc., the participants with the highest quartile of PDI were associated with 57 % reduced odds of GDM compared with women in the lowest quartile of PDI (adjusted OR 0·43; 95 % CI 0·24, 0·77; P
for trend = 0·005). An IQR increment in PDI was associated with 29 % decreased odds of GDM (adjusted OR 0·71; 95 % CI 0·56, 0·90). Findings suggest that adopting a plant-based diet during pregnancy could reduce GDM risk among Chinese women, which may be valuable for dietary counselling during pregnancy.