Abstract Objective To evaluate the efficacy of isosorbide mononitrate (IMN) for stimulating cervical ripening among pregnant women with premature rupture of membranes (PROM) at or post term. Methods A prospective randomized double‐blind, placebo‐controlled trial at Kasr El‐Ainy Hospital, Cairo, Egypt, from October 2018 to May 2019. Pregnant women at or post term with PROM and unfavorable cervix (Bishop score ≤6) were randomly assigned to receive intra‐vaginal IMN ( n  = 70) or placebo ( n  = 70) before admission for induction of labor. The main outcome was induction to the delivery interval. Data were compared between groups by t test. Results The mean ± SD duration from the initial dose of IMN/placebo to the beginning of the active phase of labor was significantly shorter in the IMN group than in the control group (9.7 ± 5.6 h vs. 12.9 ± 5.3 h). The IMN group also had a shorter time interval from induction to delivery ( P  < 0.01). There was no difference in adverse effects between the groups. Conclusion Intra‐vaginal IMN for cervical ripening in the induction of labor among pregnant with PROM at or post term was found to be effective and safe with minimal adverse effects, and good neonatal and maternal outcomes. ClinicalTrials.gov : NCT03665779. Synopsis Isosorbide mononitrate was found to be safe and effective for cervical ripening before induction of labor among pregnant women with PROM at or post term.