The purpose of this study was to examine the impact of body mass index (BMI) on breast cancer patients' self-reported health-related quality of life among patients treated with radiation therapy (RT). Women with breast cancer undergoing RT were prospectively enrolled in an Institutional Review Board-approved clinical trial between 2009 and 2012. Quality of life (QOL) assessments were collected pre-RT, during RT, and within 3 months post-RT using Euroqol (EQ-5D), MD Anderson Symptom Inventory, and functional assessment of cancer therapy-general (FACT-G). 183 breast cancer patients were enrolled, of whom 140 completed assessments at one or more time-point. After adjusting for age, chemotherapy, prior RT, type of breast surgery, and comorbidities, higher BMI remained significantly associated with worse QOL pre-RT, during RT, and post-RT in breast cancer patients. Higher BMI was strongly associated with worse overall FACT-G score on treatment and greater decline in physical well-being on treatment, which persisted after treatment. While effects on QOL of patients in the underweight and normal weight group peaked during treatment, rapidly improving by follow-up, obese patients had worse functional well-being that was more persistent at follow-up. Higher BMI was associated with worse QOL for breast cancer patients before, during, and after RT, and also was associated with reduced return to baseline QOL 3 months post-RT.