Tislelizumab combined with chemotherapy has shown significant clinical benefits in improving overall survival compared to chemotherapy alone for patients with extensive-stage small-cell lung cancer (ES-SCLC). This study aimed to evaluate the cost-effectiveness of tislelizumab plus chemotherapy versus standard chemotherapy as a first-line treatment for ES-SCLC from the US payer perspective and the perspective of the Chinese healthcare system. We conducted an economic evaluation using a Markov state-transition model, reflecting the US payer perspective and the perspective of the Chinese healthcare system. Baseline patient characteristics and essential clinical data were obtained from the RATIONALE-312 trial. The costs and utilities were derived from open-access databases and published literature. The primary outcomes measured included quality-adjusted life years (QALYs), incremental cost-effectiveness ratio (ICER), incremental net health benefit (INHB), and incremental net monetary benefit (INMB). Uncertainties in the model were addressed by probabilistic sensitivity analysis (PSA) and one-way sensitivity analysis (OWSA). In the base-case analysis, the addition of tislelizumab to chemotherapy provided an incremental gain of 0.16 QALYs at an additional cost of $7430.73, resulting in an ICER of $46,132.33 per QALY. Although above the willingness-to-pay (WTP) threshold of China of $38,042.49 per QALY, the cost-effectiveness was marginal, with an INHB of - 0.03 QALYs and an INMB of $- 1303.06. In the US, despite a slightly higher effectiveness gain of 0.28 QALYs, the increased cost of $45,157.35 resulted in an unfavorable ICER of $163,885.06 per QALY, exceeding the US WTP threshold of $150,000.00. PSA showed probabilities of cost-effectiveness of tislelizumab plus chemotherapy at 17.18% in China and 40.41% in the US. Tislelizumab combined with chemotherapy was not a cost-effective first-line treatment option for ES-SCLC in China or the US; however, the margin of cost-effectiveness was narrow.