Background: An interim analysis of the OlympiA trial found that olaparib given in the adjuvant setting can improve distant disease-free and overall survival for patients with early-stage, BRCA-mutated breast cancer; however, the cost-effectiveness of adjuvant olaparib is unknown. This study aimed to evaluate the cost-effectiveness of adjuvant olaparib in patients with early-stage, BRCA-mutated breast cancer. Methods: We used a decision-analytic model to compare outcomes of treatment with and without one year of oral olaparib after completion of systemic therapy in 42-year-old women with BRCA-mutated, early-stage breast cancer. Olaparib’s effectiveness was based on the OlympiA trial, and other model parameters were identified from the literature. We calibrated the model to reflect the 1-, 2-, and 3-year distant disease-free survival (DDFS) and overall survival (OS) observed in the OlympiA trial, and we assumed that olaparib reduced the risk of distant recurrence only in the first 3 years. Olaparib was estimated to cost $14,523 per month. Average lifetime costs were estimated from a health care system perspective in 2021 $ US, and incremental cost-effectiveness ratios (ICER) were estimated as $ per quality-adjusted life-year (QALY) gained. Costs, life-years, and QALYs were discounted by 3% annually. Results: Simulating the OlympiA trial, DDFS for the olaparib arm was 94.3% at 12 months, 90.0% at 24 months, and 87.5% at 36 months, compared to placebo with DDFS of 90.2%, 83.9%, and 80.4% respectively. Similarly, OS for the olaparib arm was 98.1%, 94.8%, and 92.0% compared to 96.9%, 92.3%, and 88.3% with placebo at 12, 24, and 36 months respectively. In the base case, adjuvant olaparib was associated with a 1.21-year increase in life expectancy and a 1.15-QALY increase at an incremental cost of $131,167 compared to placebo. The resulting ICER was about $114,500/QALY gained. At a willingness-to-pay threshold of USD$150,000/QALY, olaparib was cost effective at its current price. Results were sensitive to assumptions about the effectiveness of olaparib and its impact on quality of life. Conclusions: Adjuvant olaparib is cost-effective for women with early-stage, BRCA-mutated breast cancer at the current price of olaparib in the U.S. and at a willingness-to-pay threshold of $150,000. As such, clinicians and payers should consider adjuvant olaparib as a cost-effective option for this patient population.