INTRODUCTION: Tislelizumab, an anti-programmed cell death protein-1 antibody, was specifically engineered to minimize FcɣR macrophage binding to abrogate antibody-dependent phagocytosis. Compared with chemotherapy alone, tislelizumab plus chemotherapy may improve clinical outcomes in patients with advanced nonsquamous NSCLC (nsq-NSCLC). METHODS: In this open-label phase 3 trial (RATIONALE 304; NCT03663205), patients with histologically confirmed stage IIIB or IV nsq-NSCLC were randomized (2:1) to receive either arm A: tislelizumab plus platinum (carboplatin or cisplatin) and pemetrexed every 3 weeks (Q3Ws) or arm B: platinum and pemetrexed alone Q3W during induction treatment, followed by intravenous maintenance pemetrexed Q3W. The primary end point was progression-free survival (PFS) assessed by an independent review committee; clinical response and safety and tolerability were secondary end points. RESULTS: Overall, 332 patients (n = 222 [A]; n = 110 [B]) received treatment. With a median study follow-up of 9.8 months, PFS was significantly longer with tislelizumab plus chemotherapy compared with chemotherapy alone (median PFS: 9.7 versus 7.6 mo; hazard ratio = 0.645 [95% confidence interval: 0.462-0.902], p = 0.0044). In addition, response rates were higher and response duration was longer with combination therapy versus chemotherapy alone. Hematologic adverse events (AEs) were common in both treatment arms; the most reported AEs were grades 1 to 2 in severity. The most common grade greater than or equal to 3 AEs were associated with chemotherapy and included neutropenia (44.6% [A]; 35.5% [B]) and leukopenia (21.6% [A]; 14.5% [B]). CONCLUSIONS: Addition of tislelizumab to chemotherapy resulted in significantly prolonged PFS, higher response rates, and longer response duration compared with chemotherapy alone, identifying a new potential option for first-line treatment of advanced nsq-NSCLC irrespective of disease stage.