Aims and Objectives: To discover the current state of self-management practices and levels of self-efficacy among adults with cancer under treatment in Jeddah City, Saudi Arabia, and to identify their associated factors. Background:The prevalence of cancer and cancer treatment-related problems is increasing in Saudi Arabia. However, effective cancer care programmes are not provided in this country. Design: Cross-sectional correlational study. Methods: This study was conducted with a convenience sample of 130 adults with cancer under treatment from May 2020-August 2020 at the largest tertiary care hospital providing cancer care in the western region, Saudi Arabia. The study methods were compliant with the STROBE checklist. Data were collected by Self-Care Inventory, Strategies Used by People to Promote Health, and sociodemographic and clinical surveys. Descriptive statistics and stepwise linear regression analysis were performed. Results:The total score for self-management practices was 84.38 (SD = 13.66) and self-efficacy, as a strong associated factor of self-management, earned a total score of 104.24 (SD = 15.87). Stepwise regression analysis identified self-efficacy, age and education level as important associated variables, explaining 57.6% of the total variance in self-management practices. Significant effects of age, gender, marital status and duration of cancer on patients' self-efficacy were also found. Conclusions:The levels of self-management practices and self-efficacy to manage cancer treatment-related problems demonstrated by patients with cancer in this study were rated as a medium. Self-efficacy, age and education level were the significant factors associated with self-management practices of patients in Saudi Arabia.Age, gender, marital status and duration of cancer were identified as associated factors of self-efficacy.