Introduction: Although health behaviors are important aspects of patient care during and after cancer treatment, less attention has been paid to them. Objectives: This descriptive correlational study explored diet and physical activity and determined factors, including sociodemographic characteristics, clinical factors, self-efficacy, self-motivation, and quality of life, associated with diet and physical activity in patients with breast cancer undergoing treatment in Thailand. Methods: Participants were 108 breast cancer patients who were followed up at a university hospital in Bangkok, Thailand from December 2023 to June 2024. Self-response questionnaires, including 1) the sociodemographic characteristics questionnaire, 2) the clinical characteristics questionnaire, 3) the functional assessment of cancer therapy – breast (FACT-B) Thai version, 4) the self-efficacy scale; 5) the self-motivation questionnaire; 6) the Thai version of international physical activity questionnaire – short form (IPAQ-SF); 7) the food frequency questionnaire. Multiple linear regression was used to evaluate the effects of selected variables on diet and physical activity in the separate model using the backward method. Results: The participants' age was 55.64 (SD = 11.56). Most had university or higher education. Nearly 80% and 68.5% of participants consumed vegetables and fruits at least once daily. Two dietary patterns were identified: a healthy transitional diet and a mixed Western traditional diet. The median physical activity was 735 MET minutes/week, considering sufficiently active. Participants' mean quality of life score was 109.29 (SD = 18.98). Nutrition self-efficacy (rs = .470) and exercise self-efficacy (rs =.475) were significantly positively associated with quality of life (p < .001). The multiple linear regression showed that having lower education and receiving hormonal therapy were associated with a greater healthy diet (p < .05). Older age, higher education, and receiving breast cancer surgery were associated with lower physical activity levels (p < .05). Living in a rural house location was a factor associated with a lower quality of life (p < .05). Conclusion: Patients with breast cancer reported a healthy transitional diet and a high physical activity. Further research should involve healthcare professionals collaborating to emphasize health behaviors and create lifestyle interventions that align with guidelines to improve patient's health and quality of life.