Objective To assess the effectiveness, safety, and acceptability of post-placental insertion of GyneFix postpartum intrauterine device (PPIUD) in women undergoing cesarean section (C-section). Design Prospective cohort study. Setting Fourteen hospitals in four provinces of China. Population and sample Women who underwent C-section and consented to the post-placental insertion of GyneFix PPIUD. We enrolled 470 participants, and 400 completed the 12-month follow-up. Methods Participants were interviewed in the wards after delivery and followed up at 42 days, and months 3, 6, and 12 after delivery. Main outcome measures Pregnancy, PPIUD expulsion, serious adverse events, and continuation of PPIUD. Results Nine pregnancies were detected during the first year after GytneFix PPIUD insertion, 7 were due to device expulsion and 2 occurred with PPIUD in situ. The Pearl Indices (PI; pregnancy per 100 women-years) for overall 1-year pregnancy rate and pregnancies with IUD in situ were 2.32 (95% CI: 1.06–4.40) and 0.51 (95% CI: 0.06–1.86), respectively. The 1-year expulsion PI was 8.25 (95% CI: 5.63–11.63). The expulsion PI was significantly higher in the first 6 months (12.78, 95% CI: 8.42–18.60) than the second 6 months (2.82, 95% CI: 0.92–6.58). The cumulative 1-year continuation rate was 86.56% (95% CI: 83.32–89.79). We did not identify any patient with insertion failure, uterine perforation, pelvic infection, or excess bleeding due to GyneFix PPIUD insertion. Conclusions Post-placental insertion of GyneFix PPIUD is effective, safe, and acceptable for women undergoing C-section. An ultrasound scan during the first 6 months after PPIUD insertion is recommended to identify any unrecognized expulsions.