BackgroundUp to one-third of women globally experience giving birth as traumatic, which can lead to postpartum post-traumatic stress disorder. Postpartum post-traumatic stress disorders have significant health consequences for the mother, child, and other family members. Although it has tragic health impacts, little is known about this problem in the study area.ObjectivesThe study aimed to assess the prevalence of postpartum posttraumatic stress disorder and associated factors among postnatal mothers.MethodsA community-based cross-sectional study was employed among 635 mothers in the first year after childbirth by using simple random sampling techniques from March 20 to April 20, 2024 in West Arsi zone, Ethiopia. Face-to-face interviewers administered structured questionnaires that were used to collect the data. The collected data were cleaned, coded, and entered into Epidata and exported to Statistical Package for Social Sciences for further analysis. Both bivariate and multivariable analyses were done by using binary logistic regression.ResultOut of 635, 624 postnatal mothers participated in the study, for a response rate of 98.27%. The prevalence of postpartum post-traumatic stress disorder was 21.60% (95% CI: 18.40%, 24.87%). Primiparous mothers (AOR = 2.26, 95% CI: 1.38, 3.70), have no antenatal care follow-up (AOR = 2.48, 95% CI: 1.47, 4.20), cesarean section delivery (AOR = 2.86, 95% CI: 1.50, 5.61), instrumental delivery (AOR = 3.06, 95% CI: 1.75, 5.34), maternal morbidity (AOR = 2.94, 95% CI: 1.71, 5.05), and postpartum intimate partner violence (AOR = 7.43, 95% CI: 4.53, 12.20) were the identified factors.Conclusion and recommendationAs identified, one out of five mothers had postpartum posttraumatic stress disorder. Thus, healthcare providers should focus on identified factors like cesarean section and instrumental deliveries while counseling, as this enhances the mothers’ psychological readiness. In addition, the West Arsi Zonal Health Office should develop effective strategies to alleviate the problem by focusing on the identified factors.