To evaluate the association between serum folate levels during pregnancy and prenatal depression and the extent to which obesity may modify this relationship. This secondary data analysis leveraged data from a previous study of pregnant Kaiser Permanente Northern California participants who completed a survey and provided a serum sample between 2011 and 2013. Serum folate was assessed using the Center for Disease Control’s Total Folate Serum/Whole Blood Microbiological Assay Method. A score of 15 or greater on the Center for Epidemiologic Studies Depression Scale was defined as prenatal depression. We used Poisson regression to estimate risk of prenatal depression given prenatal serum folate status (low/medium tertiles vs. high tertile) in the full sample and in subsamples of women with pre-pregnancy body mass index in the (a) normal range and (b) overweight/obese range. Of the sample, 13% had prenatal depression. Combined low/medium folate tertiles was associated with prenatal depression (adjusted relative risk [aRR]?=?1.97, 95% confidence interval [CI]: 0.93-4.18), although results did not reach statistical significance. This relationship was stronger among women with overweight/obesity than women with normal weight (aRR: 2.61, 95% CI: 1.01-6.71 and aRR: 1.50, 95% CI: 0.34-6.66, respectively). Results suggest an association between lower pregnancy folate levels and prenatal depression that may be stronger among women with overweight or obesity. Future studies need to clarify the temporal sequence of these associations.