Background: The concept of financial toxicity (FT) has been used to describe the unanticipated economic damage potentially caused by cancer treatment. Currently, it has been associated with decreased health-related quality of life (QoL), treatment nonadherence and increased risk of mortality. Even in publicly funded environments as Brazilian Unified National Health System, factor such as income loss or out-of-pocket costs could become an issue for cancer care. The aim of this study was to examine the FT related to the breast cancer diagnosis and treatments in a public Brazilian tertiary hospital through Comprehensive Score for Financial Toxicity (COST) instrument. The COST is a 12-item survey scored from 0 to 44, with lower scores reflecting worse financial well-being. Methods: This cross-sectional study was conducted among breast cancer patients in active treatment for at least 6 months attending medical oncology out-patient department. Socio-demographic, clinical characteristics and a simplified assessment of QoL were collected as well as responses to COST questionnaire. Descriptive statistic and analysis of correlations between quantitative variables and scores of QoL and COST were performed. Results: A total of 100 female patients were included with median age of 53 years old. The mean time from initial diagnosis was of 49.6 months and clinical stage IV represented 46% of all cases. Prior to the cancer treatment, 48 (48%) of the patients had their own job as a main source of income. However, after the cancer diagnosis, only 9 (9%) patients keep working, 36 (36%) needed to use savings and 29 (29%) applied for a bank loan. To adjust their finances, 66 (66%) patients reduced spending on leisure. The median COST score was 18.5 (ranged from 4 to 41). According to the age, patients with ≤ 45 years old were at higher risk of suffering FT (p = 0.01). There was also an association between the COST score and QoL since the higher the FT, the lower the QoL (p < 0.001). Conclusions: Patients with breast cancer are at risk of losing their work capacity, become financially dependent of other, and suffering a reduction in QoL as a result of FT. Mostly younger patients are at greater risk of FT. Despite a universal health-model, this study highlights a substantial proportion of patients who experienced FT. Therefore, further studies confirming these findings in larger groups are warranted to develop better strategies and policies to mitigate this form of toxicity.