Objective: This study evaluated the cost-effectiveness of sacituzumab govitecan (SG) compared with single-agent chemotherapy of the physician’s choice (TPC) from the perspective of Taiwan’s National Health Insurance. Methods: A partitioned survival model was developed to assess outcomes in patients with metastatic triple-negative breast cancer (mTNBC). Clinical data were derived from the ASCENT trial, while direct medical costs were obtained from Taiwan’s National Health Insurance Administration (NHIA). Utility values were taken from published literature. The primary outcome was the incremental cost-effectiveness ratio (ICER), expressed as cost per quality-adjusted life year (QALY) gained. One-way and probabilistic sensitivity analyses were performed to examine parameter uncertainty and test the robustness of the results. Results: In the base-case analysis, SG was associated with an incremental cost of USD 121,836 per QALY gained—exceeding Taiwan’s willingness-to-pay (WTP) threshold of USD 102,120. One-way sensitivity analyses indicated that SG drug cost was the primary driver of ICER variability. Probabilistic sensitivity analysis showed that reducing the price of SG by 50% increased the likelihood of cost-effectiveness. Conclusions: From the NHIA perspective, SG is not cost-effective for patients with advanced or metastatic TNBC at its current price. Substantial price reductions would be required for SG to become cost-effective under the WTP threshold of USD 102,120 per QALY.