Background: Exposure to secondhand smoke (SHS) during pregnancy has been linked to adverse maternal and perinatal outcomes. Preterm birth is one of the most critical complications. However, the evidence of an association between SHS exposure and preterm birth remains inconclusive. Objectives: our aim was to identify the prevalence of exposure to secondhand smoke among pregnant women and investigate the association between exposure to SHS and preterm birth. Methods: A cross-sectional study was conducted in Madinah, Saudi Arabia, from October to December 2024. A self-administered questionnaire was distributed among women in the postnatal ward to assess sociodemographic and pregnancy-related variables and exposure to SHS. Results: A total of 469 women were included in the study. About 33.7% of the women reported that their husbands currently smoked tobacco, with 51% smoking indoors; 21% of the women did not impose smoking restrictions at home. Pregnant women exposed to SHS had more than twice the odds of preterm birth compared to unexposed women (OR = 2.09; 95% CI: 1.06–4.13; p = 0.03). Conclusions: SHS exposure increased the risk of preterm birth among pregnant women in Madinah. These findings emphasize the essential need for preventive strategies to diminish SHS exposure in residential and public environments.