Abstract Aims/hypothesis Season of birth as a surrogate for potential environmental exposure during fetal development and early postnatal lifehasshownaninconsistentassociationwithadult type 2 diabetes in white populations living in high-latitude regions. The present study aimed to examine the association between birth seasonality and risk of adult type 2 diabetes in Chinese individuals livingacross wideregions of low latitude and lower to middle latitude. Methods Participants fromthe ChinaKadoorieBiobank were enrolled during 2004 –2008 and followed up until 31 December 2013. After excluding participants with cancer, heartdisease,strokeanddiabetesatbaseline,thepresentstudy included189,153menand272,058womenaged30–79years. We used multivariable Cox proportional hazards model to estimate the HR and 95% CI. Results During a median follow-up of 7.2 years (3.3 million person-years), we documented 8784 incident cases of type 2 diabetes. In the whole cohort, compared with summer-born participants, the adjusted HRs (95% CIs) were 1.09 (1.02, 1.16), 1.08 (1.02, 1.15) and 1.09 (1.02, 1.15) for those who were born in Spring, Autumn and Winter, respectively. The association was consistent in both men and women and across subgroupsdefinedbyresidenceandlifestylefactorslaterinlife. Conclusions/interpretation In this large prospective study, participants born in summer had a lower risk of adult type 2