Background: Breast cancer (BC) is the most common type of cancer among Iranian women. The cost of breast cancer treatment is high, and many families struggle to afford it. Objectives: This study was conducted with the aim of determining the out-of-pocket (OOP) health expenditure and factors Methods: This study was conducted in a cross-sectional descriptive-analytical way. Based on the inclusion criteria, the health expenditure of 82 patients with BC were collected. The data was gathered from Shahid Jalil Hospital, affiliated with Yasuj University of Medical Sciences. It includes inpatient and outpatient information from the Iran Health Insurance Organization, as well as patient-declared costs in 2022. The study data analyzed using descriptive statistics methods including frequency, percentage, mean and standard deviation, and stepwise linear regression to investigate the effect of variables on the amount of OOP health expenditure in SPSS 21 software. Results: The OOP expenses for BC patients accounted for 32.89% of the total direct medical expenses. Of the OOP costs, 47.18% were attributed to drug expenses, 16.19% to laboratory costs, 11.74% to imaging expenses, 11.20% to visit costs, 8.40% to hospitalization expenses, 2.84% to doctor's services, and 2.45% to physical therapy-related expenses. Factors such as age, place of residence, occupation, education, and household income were among the factors that had a significant effect on OOP payments (P < 0.05). Marital status, housing situation, and social coverage had no significant effect on patients' OOP payments (P> 0.05). Conclusions: BC patients incur a lot of expenses, and about 32.89% of these expenses are OOP payments. It is essential for insurance organizations to increase their coverage, while also requiring additional support from the government for patients with breast cancer in obtaining necessary medication and medical supplies.