BACKGROUND AND AIM[|]Women are more prone to mental health problems at postpartum because of biological, emotional, and social changes during this period. Evidence regarding the effect of temperature on postpartum depression (PPD) is extremely limited. We aimed to examine the associations between postpartum temperature exposure and PPD.[¤]METHOD[|]We conducted a retrospective cohort study using data from Kaiser Permanente Southern California electronic health records from 01/01/2008 through 12/31/2018. PPD was first assessed using the Edinburgh Postnatal Depression Scale (score≥10) during the first year of the postpartum period, and further identified by using both diagnostic codes and prescription medications. Historical daily ambient temperatures were obtained from the 4-km resolution gridMET dataset, and linked to participants’ residential addresses at delivery. Postpartum temperature exposures were measured by calculating various temperatures metrics during the period from delivery to PPD diagnosis date. A time-to-event approach with a discrete-time logistic regression was applied to estimate the association between temperature and time to PPD. Effect modification by maternal characteristics and other environmental factors was examined.[¤]RESULTS[|]There were 45,073 (10.49%) PPD cases among 429,839 pregnancies. Increased PPD risks were positively associated with exposure to higher mean temperature (adjusted odds ratio [aOR] per interquartile range increment: 1.07, 95% confidence interval [CI]: 1.05-1.09) and diurnal temperature range (aOR=1.08, 95%CI: 1.06-1.10); the associations were stronger for maximum temperature compared to minimum temperature. African American, Asian, or Hispanic mothers, and mothers ≥25 years of age were more susceptible to temperature-related PPD risk compared to their counterparts. We also observed higher effect of temperature on PPD among mothers exposed to higher air pollution levels, lower green space levels or with lower air conditioning penetration rates.[¤]CONCLUSIONS[|]Maternal exposure to higher temperature and diurnal temperature variation during the postpartum period was associated with an increased risk of PPD. Effect modification by maternal age, race/ethnicity, air pollution, green space and air conditioning penetration was identified.[¤]