Background: The diagnosis and treatment of cancer can be a traumatic and life-changing event for patients, leading to increased levels of stress and psychological trauma, which can significantly impact their quality of life. Objective: This study aimed to identify the prevalence of post-traumatic stress disorder (PTSD) among adult cancer patients in Oman, determine predictors influencing the development of PTSD and investigate its relationship with social support and quality of life. Method: A descriptive correlational cross-sectional study was conducted involving 343 adult patients with cancer undergoing treatment at three tertiary care centers in Oman, which were Sultan Qaboos Comprehensive Cancer Care and Research Centre, Royal Hospital and Sultan Qaboos University Hospital. PTSD was assessed using the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (PTSD Checklist for DSM-5; PCL-5), while perceived social support was measured using the Multidimensional Scale of Perceived Social Support (MSPSS). Quality of life was assessed using the European Organization for Research and Treatment of Cancer (EORTC QLQ-C30) scale. Data were analyzed by using The Statistical Package for Social Science (SPSS) software version 25. To assess PTSD prevalence, symptom cluster criteria was used and compared against a scoring threshold of 32 and 33.The relationship between social support and PTSD was examined using simple logistic regression, while the Mann-Whitney U test was utilized to determine if quality-of-life scores differed significantly CANCER-RELATED POST-TRAUMATIC STRESS DISORDER between patients with and without PTSD. Additionally, multiple logistic regression was employed to identify predictors of PTSD among Omani adult cancer patients. Result: The study showed that the prevalence of PTSD among Omani adult patients with cancer was 27.4%. Importantly, a significant negative association was observed between social support and PTSD (odds ratio [OR] = 0.95; 95% confidence interval [CI] = 0.93 - 0.97; p 0.001), indicating that higher levels of social support acted as a protective factor against the development of PTSD in this population. Patients diagnosed with PTSD exhibited significantly lower mean ranks for global health status/quality of life (mean rank = 97.3) in comparison to patients without PTSD (mean rank = 200.2) (p 0.001). Similarly, patients with PTSD also demonstrated lower mean ranks for the total functional scale (mean rank = 76.7) compared to those without PTSD (mean rank = 208.0) (p 0.001). Conversely, patients with PTSD had a higher mean rank for the total symptoms scale (mean rank = 253.5) in contrast to patients without PTSD (mean rank = 140.4) (p 0.001). Furthermore, several predictors of PTSD were identified within this population. A shorter time since diagnosis (OR = 0.98, 95% CI = 0.97-0.99, p = 0.017), absence of a psychiatric referral (OR = 0.46, 95% CI = 0.23-0.96, p = 0.04), and inadequate social support (OR = 0.96, 95% CI = 0.94-0.98, p 0.001) were significantly associated with an increased likelihood of developing PTSD among patients with cancer. Conclusion: The study revealed that PTSD is a significant mental health issue among patients with cancer, with a prevalence rate of 27.4%. The study highlights the critical role of social support in reducing the risk of PTSD and emphasizes the need for routine assessment and psychiatric referral to improve the quality of life of patients with cancer especially those who are newly diagnosed.