OBJECTIVES: Trastuzumab deruxtecan (T-DXd) was recently recommended by the Committee for Medicinal Products for Human Use as a treatment for adult patients with unresectable or metastatic HER2-positive breast cancer, who had received a prior anti-HER2-based regimen. In our study, we evaluated the cost-effectiveness of T-DXd compared with ado-trastuzumab emtansine (T-DM1) for this indication in Finland. METHODS: A three-state partitioned survival analysis model was developed with a payer's perspective. Time to event data from the DESTINY-Breast03 (DB-03) trial were extrapolated over a lifetime horizon either directly-for progression-free survival and time to treatment discontinuation-or using an alternative approach utilizing long-term T-DM1 survival data and DB-03 data-for overall survival. Discount rates of 3% were applied for costs and effects. Inputs were sourced from the Medicinal Products Database from Kela, Helsinki University Hospital service price list, Finnish Medicines Agency assessments, clinical experts, and DB-03. Sensitivity analyses were performed to characterize and demonstrate parameter uncertainties in the model. RESULTS: Total quality-adjusted life years (QALYs) and life years (LYs) gained for T-DXd compared with T-DM1 were 1.93 and 2.56, respectively. Incremental costs for T-DXd compared with T-DM1 were €106,800, resulting in an ICER of €55,360 per QALY gained and an ICER of €41,775 per LY gained. One-way sensitivity analysis showed the hazard ratio of T-DXd vs T-DM1 for OS was the most influential parameter. The probabilistic sensitivity analysis showed similar results to the base case. CONCLUSIONS: T-DXd is cost-effective based on surrogate WTP thresholds of €72,000 and €139,000 per QALY.