Background The aim of this study was to evaluate the relationship between vitamin D and postpartum depression in reproductive-aged Iranian women. Methods and Results This study was conducted on 120 women (60 with postpartum depression and 60 without) in Izeh, Iran. A socio-demographic questionnaire and Beck Depression Scale were used for data collection. The ELISA method was used for measuring 25-OH vitamin D (ng). The participants were classified according to their vitamin D level as follows: 25-OH-D < 10ng/ml considered as severe deficiency, 10–20n g/ml as moderate insufficiency, 20–30 ng/ml as mild insufficiency and >30ng/ml as normal. Data were analyzed using the independent t-test or Mann-Whitney test, chi-square and logistic regression test. The mean level of vitamin D of women with postpartum depression was lower than that in normal women (16.89±7.05 vs. 21.28±7.13, p=0.001). More than 53% of women with postpartum depression had vitamin D <20 ng/ml compared to 31.7% of women with no depression (p=0.005). Moreover, 16.7% of women with postpartum depression had vitamin D < 10ng/ml compared to only 6.7% in the normal group (p = 0.005). Women with vitamin D less than 20ng/ml compared to vitamin D > 20ng/ml were 3.30 times more likely to have postpartum depression (OR: 3.3, CI: 1.32–8.24, p= 0.01). Discussion There is a significant relationship between a low level of vitamin D and postpartum depression among reproductive-aged Iranian women. Health policy makers should pay attention to the measuring vitamin D level as one of the primary tests of pregnant women.