BackgroundNatural disasters cause long term psychological consequences, especially post-traumatic stress disorders. It has been regarded as the most prevalent of psychiatric disorders after a natural disaster. The purpose of this study is to estimate the prevalence of Post-Traumatic Stress Disorder (PTSD) and determine its associated factors in adult survivors three years after the 2015 Nepal earthquake.MethodsA cross-sectional descriptive design was used where 1076 adults within the age range of 19–65 were randomly selected and interviewed from four adversely affected districts due to the 2015 earthquake. Instruments included a demographic questionnaire, an earthquake exposure questionnaire, the Oslo Social Support Scale (OSSS), and the Post-traumatic Stress Disorder Checklist-Civilian Version (PCL-C). Descriptive and inferential statistics were applied using Statistical Package for Social Science (SPSS) Version 16 for data analysis.ResultsThe prevalence of PTSD among earthquake survivors was 18.9%. The multivariate logistic regression showed that gender, ethnicity, education, occupation, social support and severity of damage to house and property were significantly associated with PTSD. Odds of having PTSD was 1.6 times higher among females (AOR = 1.6, 95% CI: 1.1–2.3) and nearly 2 times higher amongst illiterate survivors (AOR = 1.9, 95% CI: 1.2–2.8). Participants from the Janajati ethnic group and those who had a business occupation had a 50% lower risk of having PTSD. Around 39% of the participants had moderate social support and had 60% lower odds of having PTSD compared to those with poor social support (AOR = 0.4, 95%CI: 0.2–0.5,p< 0.001). Participants with medium and very high-level damage to personal property were more likely to have PTSD.ConclusionPost-traumatic stress disorder remained prevalent amongst survivors three years after the 2015 Nepal Earthquake. It is important to provide psychological and social support for survivors to decrease the health burden from PTSD. Socio-demographic characteristics such as females, farmers, those survivors who endured significant personal property damage were at higher risk.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12888-023-04836-3.