Background: - Posttraumatic stress disorder (PTSD) is one of the most common mental health problems that military personnel encounter. It could be life-long and affect every aspect of military personnel, including their mental and physical health, family and social interactions, and their work. However, in Ethiopia, the magnitude and its associated factors has not been well investigated. Objective: This study aimed to determine the prevalence of posttraumatic stress disorder and its associated factors among military personnel, who were admitted Northwest Command level three military hospital, Bahir Dar, Northwest, Ethiopia, 2022. Methods: An institution-based cross-sectional study was conducted from June 21 to July 21, 2022 Northwest command level three military hospital. Computer generated simple random sampling technique was used to select a total of 627 participants. Posttraumatic stress disorder military version check list was used to assess the prevalence of Posttraumatic stress disorder. The data were coded and entered into EPI data version 4.46 then exported to Statistical package for social science version 25.0. Descriptive, bivariate and multivariate binary logistic regressions with odds ratios and 95% confidence interval were employed. The level of significance of association was determined at p-value < 0.05. Results; A total of 612 respondents participated with response rate of 97.6%. The prevalence of post-traumatic stress disorder in this study was 21.9 %( 95 % CI: 18.6, 25.2). In multivariable regression female sex (AOR=2.3, 95CI; 1.3, 3.87), combat personnel (AOR=2.75, 95CI; 1.44, 6.36), handling dead bodies (AOR=2.5, 95CI,1.24,5.02), having 4-5 deployment,(AOR=2.94, 95CI, 1.63, 5.32), having ≥6 deployment (AOR=3.4, 95CI, 1.95, 6.17), low resilience coping (AOR= 2.02,95 CI; 1.16, 3.53), poor social support (AOR=2.46,95CI,1.39,4.35), very high combat exposures( AOR= 4.8,95CI, 2.03,11.93), and depression (AOR= 2.8,95 CI,1.68, 4.67) were significantly associated with PTSD. Conclusion: The prevalence of post-traumatic stress disorder was found to be high in military personnel. Female sex, poor social support, low resilience copers, handling dead bodies, having ≥4 deployment, high combat exposure and depression were significantly associated with posttraumatic stress disorder. Northwest command level three military hospital better to emphasize on early identification and intervention on individuals with these problems. Key Words; Military personnel, PTSD, Prevalence