Background: Immune checkpoint inhibitors plus chemotherapy (chemo) improve antitumor efficacy as first-line treatment in patients with esophageal squamous cell carcinoma (ESCC). However, the optimal target population for this combination therapy remains uncertain. At ESMO Asia Congress 2022, we presented results from the interim analysis of the phase 3 study of serplulimab (anti-PD-1 antibody) plus chemo as first-line treatment for PD-L1–positive (CPS ≥1) ESCC patients. Here we report the results from an updated analysis, which is also the final analysis of overall survival (OS). Methods: In this randomized, double-blind, multicenter phase 3 study, patients with histologically confirmed locally advanced or distantly metastatic, PD-L1–positive ESCC who had no prior systemic antitumor therapy were randomized 2:1 to receive serplulimab 3 mg/kg or placebo plus chemo (5-FU + cisplatin) intravenously Q2W. Randomization was stratified by PD-L1 expression level (CPS ≥10 vs CPS < 10), age (≥65 vs < 65 years), and disease status (locally advanced vs distantly metastatic). The primary endpoints were IRRC-assessed progression-free survival (PFS) per RECIST v1.1 and OS. Secondary endpoints included other efficacy measures, safety, quality of life, and biomarkers. Results: Between June 19, 2019 and December 17, 2021, 976 patients were screened and 551 were randomized (serplulimab-chemo, n = 368; placebo-chemo, n = 183). At this updated analysis, median follow-up duration was 24.2 months. Median OS was significantly longer in the serplulimab-chemo group than the placebo-chemo group (14.6 vs 11.2 months; hazard ratio [HR] 0.70, 95% CI 0.57–0.86; P = 0.0006). IRRC-assessed median PFS per RECIST v1.1 was prolonged with the addition of serplulimab (6.5 vs 5.3 months; HR 0.58, 95% CI 0.47–0.72). Efficacy improvements were also observed in confirmed objective response rate (58.7% vs 42.1%) and duration of response (median, 7.1 vs 4.6 months) as assessed by IRRC per RECIST v1.1. Grade ≥3 treatment-related adverse events were reported in 203 (53.1%) and 82 (48.8%) patients in the respective groups. Incidence of grade ≥3 immune-related adverse events was higher in the serplulimab-chemo group compared to the placebo-chemo group (9.2% vs 3.0%). Fifteen deaths (3.1% in the serplulimab-chemo group and 1.8% in the placebo-chemo group) that might be related to study treatment were reported. Conclusions: With another 9.3 months of follow-up, the encouraging efficacy and manageable safety of serplulimab plus chemo as first-line treatment in patients with PD-L1–positive advanced ESCC were maintained, further supporting the development of serplulimab plus 5-FU and cisplatin for previously untreated ESCC patients. Clinical trial information: NCT03958890.