Background: There is a paucity of research on the cost of breast cancer (BC) treatment from the patient’s perspective in Vietnam.Methods: Individual-level data about out-of-pocket (OOP) expenditures on use of services were collected from women treated for BC (n=202) using an online survey and a face-to-face interview at two tertiary hospitals in 2019. Total expenditures on diagnosis and initial BC treatment were presented in terms of the mean, standard deviation, and range for each type of service use. A generalised linear model (GLM) was used to assess the relationship between total cost and socio-demographic characteristics.Results: 19.3% of respondents had stage 0/I BC, 68.8% had stage II, 9.4% had stage III, none had stage IV. The most expensive OOP elements were targeted therapy with mean cost equal to 649.5 million VND ($28,025) and chemotherapy at 36.5 million VND ($1,575). Mean total OOP cost related to diagnosis and initial BC treatment (excluding targeted therapy cost) was 61.8 million VND ($2,667). The mean OOP costs among patients with stage II and III BC were, respectively, 66% and 148% higher than stage I.Conclusions: BC patients in Vietnam incur significant OOP costs. The cost of BC treatment was driven by use of therapies and presentation stage at diagnosis. It is likely that OOP costs of BC patients would be reduced by earlier detection through raised awareness and screening programs and by providing a higher insurance coverage rate for targeted therapy.