Background: The prevalence of malnutrition in cancer patients ranges from 30 to 60%. While it is known that malnutrition is prevalent among cancer patients, the relationship between undernutrition and quality of life among breast cancer patients has not been adequately explored. Therefore, the present study was aimed at assessing the association between undernutrition and quality of life among Breast Cancer patients under treatment in Addis Ababa, Ethiopia. Methods: A cross-sectional study on breast cancer patients under treatment at the outpatient and in-patient departments of oncology centers of two tertiary hospitals in Addis Ababa – Tikur Anbessa Specialized Hospital (TASH) and St. Paul Millennium Medical College (SPHMMC) oncology was conducted from May 12 to August 26,2020. Nutritional status was assessed using Subjective Global Assessment (SGA) screening tool. Quality of life was assessed using the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire Cancer 30 (EORTC QLQ C30) a standard quality of life measurement scale for cancer patients. To determine the relationship between quality of life scores and nutritional status multivariable linear regression was fitted. Results: A total of 411 breast cancer patients with mean age of 44.4 (± 11.47) years. And 393 (95.6%) of them female were included in the study. A high prevalence of moderate 127 (30.9%) and severe 106 (25.8%) malnutrition was observed. Moderate (β = −9.21 CI (− 14.59, − 4.67)) and severe (β = −17.81 CI (− 16.6, 2 − 2.91)) malnutrition were negatively associated with the overall quality of life. Malnutrition also showed negative associations with all domains of functional status (P < 0.05) and strong positive associations with symptom scores covered in the EORTC QLQ C-30 (P < 0.05). Conclusion: This study indicated that malnutrition is a major problem among breast cancer patients and the nutritional status breast cancer patients was related to their quality of life….