Background: Many women experience sexual dysfunction following childbirth but this has not been well investigated in Iran. The aim of this study was to evaluate women’s sexual function in the postpartum period in Iran. It also sought to determine predicting factors associated with their sexual function. Methods: This was a cross-sectional study among 380 postpartum women attending 10 urban health centers in Ilam province in southwestern Iran. Participants were selected using random cluster sampling. Data was collected using the female sexual function index (FSFI) and a checklist of socio-demographic and maternal status for each of the women. Sexual dysfunction was classified according to an FSFI score of ≤ 28. Data were analysed using SPSS version 22. Results: The majority of participants (76.3%) had sexual dysfunction. Primiparity (adjusted odds ratio (aOR): 1.78 (95% Confidence Interval (CI): 1.11, 2.94); P = 0.006) and exclusive breastfeeding (aOR: 2.47 (95% CI: 1.21, 5.03); P = 0.012) were associated with increased odds of experiencing sexual dysfunction in the postpartum period. Other factors such as age, type of delivery, education, time since delivery and family income did not predict women’s postpartum sexual function. Conclusion: This study confirmed findings of previous studies on factors that may have an adverse effect on new mothers’ sexual function in the postpartum period. However the effect of type of delivery on postpartum sexual function remains unclear.