Breast cancer is the most frequent cancer in women in Africa and contributes to premature death and poor quality of life. This study aimed to determine the validity, reliability, and psychometric properties of the Swahili version of EORTC QLQ-BR45 among women with breast cancer in Tanzania. A cross-sectional study design with non-probability convenience sampling was employed. Data were collected in two tertiary hospitals and one national cancer institute; 414 participants completed the EORTC-QLQ-C-30 and EORTC-QLQ-BR45. The reliability of QLQ-BR45 was measured using Cronbach's alpha and McDonald's Omega coefficients. The factor structure of EORTC QLQ-BR45 was assessed using confirmatory factor analysis. The internal consistencies for the five dimensions were all above 0.7 indicating satisfaction, except for systemic therapy side effects with a marginal value of 0.594 and significant correlations between the dimensions of QLQ-C30 and BR45. The final model fit well to the data, with the comparative fit index = 0.953, Tucker-Lewis index = 0.947, root mean square error of approximation = 0.041 (90% CI: 0.035, 0.046), and standardized root mean square residual = 0.072. In conclusion, the QLQ BR45 Swahili version displayed good reliability, validity, and psychometric properties and can be used in Swahili-speaking Sub-Saharan countries.