The standard chemotherapy for squamous NSCLC (sqNSCLC) includes platinum plus gemcitabine. Sintilimab, an anti–programmed cell death protein 1 antibody, plus platinum and gemcitabine (GP) has revealed encouraging efficacy as first-line therapy for sqNSCLC in a phase 1b study. We conducted a randomized, double-blind, phase 3 study to further compare the efficacy and safety of sintilimab with placebo, both in combination with GP. ORIENT-12, a randomized, double-blind, phase 3 study, was conducted at 42 centers in the People’s Republic of China (ClinicalTrials.gov, number NCT03629925). Patients with locally advanced or metastatic sqNSCLC and without EGFR-sensitive mutations or ALK rearrangements were enrolled in the study. The stratification factors included clinical stage, choice of platinum, and programmed death-ligand 1 tumor proportion score. The patients, investigators, research staff, and sponsor team were masked to treatment assignment. Eligible patients were randomized 1:1, using an integrated web-response system, to receive sintilimab 200 mg or placebo plus GP every 3 weeks for four or six cycles, followed by sintilimab or placebo as maintenance therapy until disease progression or 2 years. The primary end point was progression-free survival (PFS), assessed by an independent radiographic review committee. Between September 25, 2018 and July 26, 2019, a total of 543 patients were screened, of whom 357 patients were randomized to the sintilimab-GP group (n = 179) and the placebo-GP group (n = 178). After a median follow-up period of 12.9 months, sintilimab-GP continued to reveal a meaningful improvement in PFS than placebo-GP (hazard ratio = 0.536 95% confidence interval: 0.422–0.681, p < 0.00001). Treatment-emergent adverse events of grade 3 or worse occurred in 86.6% patients in the sintilimab-GP group and in 83.1% in the placebo-GP group. The incidence of treatment-emergent adverse event leading to death was 4.5% and 6.7% in the two treatment groups, respectively. Regarding PFS, sintilimab plus GP reveals clinical benefit than GP alone as first-line therapy in patients with locally advanced or metastatic sqNSCLC. The toxicity was acceptable, and no new unexpected safety signals were observed.