Retrospective studies suggest that early time-of-day (ToD) infusions of immunochemotherapy may improve efficacy. However, prospective randomized controlled trials are needed to validate it. In this randomized phase 3 LungTIME-C01 trial, 210 patients with treatment naive stage IIIC–IV non-small cell lung cancer (NSCLC) lacking driver mutations were randomly assigned in a 1:1 ratio to either an early or late ToD group, defined by the administration of the first four cycles of an anti-PD-1 agent before or after 15:00 h. The primary endpoint was progression-free survival (PFS), while secondary endpoints included overall survival (OS) and objective response rate (ORR). After a median follow-up of 28.7 months, the median PFS was 11.3 months (95% confidence interval (CI) = 9.2–13.4) in the early ToD group and 5.7 months (95% CI = 5.2–6.2) in the late ToD group, corresponding to a hazard ratio (HR) for earlier disease progression of 0.40 (95% CI = 0.29–0.55; P< 0.001). The median OS was 28.0 months (95% CI = not estimable (NE)–NE) in the early ToD group and 16.8 months (95% CI = 13.7–19.9) in the late ToD group, corresponding to an HR of an earlier death of 0.42 (95% CI = 0.29–0.60; P< 0.001). Treatment-related adverse events were consistent with the established safety profile, with no new safety signals observed. No significant differences in immune-related adverse events were observed between the two groups. Over the first four cycles, morning circulating CD8⁺ T cells increased in the early ToD group, whereas they declined in the late ToD group (P< 0.001). Furthermore, the ratio of activated (CD38⁺ HLA-DR⁺) versus exhausted (TIM-3⁺PD-1⁺) CD8⁺ T cells was higher in the early ToD group (P< 0.001) compared with the late ToD group (P< 0.001). In summary, our study indicates that early ToD immunochemotherapy substantially improves PFS and OS and is associated with enhanced antitumor CD8⁺ T cell characteristics compared with late ToD treatment.