BACKGROUND: Triple-negative breast cancer accounts for 15% of all breast cancer cases, and it has a lower survival rate and higher incidence of early recurrence, particularly during the first 10 years after diagnosis. OBJECTIVE: This study aimed to estimate the cost and burden of triple-negative breast cancer among the female population in 2019 in Portugal from a societal perspective. METHODS: The prevalence of triple-negative breast cancer was calculated using a cumulative incidence model on the basis of national epidemiological data. The burden of disease was expressed as disability-adjusted life years, including the years lost due to disability and years of life lost. Healthcare resource utilization was quantified with input from an expert panel, and costs were estimated on the basis of diagnosis-related groups. Indirect costs were established following the human capital approach and supported by inputs from an expert panel. RESULTS: Considering a prevalence of 7052 cases of triple-negative breast cancer in 2019, the expert panel confirmed that approximately 24%, 29%, 28% and 19% of the patients were in stages I, II, III and IV, respectively. The burden of this disease in Portugal was estimated at 22,566 disability-adjusted life years per year, 94% of which resulted from premature deaths. The total annual cost was equal to €50,351,934, with direct and indirect costs representing 56% and 44%, respectively. The average cost per patient with triple-negative breast cancer was €7140. Direct costs accounted for €28 million and were associated mainly with triple-negative breast cancer locoregional stage treatment and follow-up (65%). Indirect costs represented €22 million and were largely linked to withdrawal from the job market (94%). CONCLUSION: Triple-negative breast cancer is an impactful disease with high humanistic and economic costs at the national level. The high mortality and low survival rates of this subtype mean that most disability-adjusted life years are due to years of life lost rather than years lost due to disability. Its prevalence is greater among women aged 45-49 years, suggesting a considerable burden regarding labour absenteeism and withdrawal from the job market.