BACKGROUND: In updated analyses from the phase III POSEIDON study, after a median follow-up of >5 years, tremelimumab plus durvalumab and chemotherapy (T + D + CT) showed durable long-term overall survival (OS) benefit versus CT alone in first-line metastatic non-small-cell lung cancer (mNSCLC). In this article, we report the associations of tumor mutational burden (TMB) with outcomes of D with or without T in combination with CT versus CT alone. PATIENTS AND METHODS: A total of 1013 patients with EGFR/ALK wild-type mNSCLC were randomized (1 : 1 : 1) to T + D + CT, D + CT, or CT, stratified by programmed cell death-ligand 1 (PD-L1) tumor cell (TC) expression ≥50% versus <50%, disease stage (IVA versus IVB) and histology (squamous versus nonsquamous). Patient subgroups were defined by a range of blood TMB (bTMB) values, including at a prespecified cut-off of 20 mutations (mut)/megabase (Mb) and across further subdivisions by PD-L1 TC expression ≥1% or <1% and by tissue TMB (tTMB) values. RESULTS: At the primary OS data cut-off (12 March 2021), at each bTMB or tTMB cut-off, the magnitude of OS benefit appeared greater among patients in the bTMB- or tTMB-high subgroups for the T + D + CT arm versus the CT arm but was similar between subgroups for the D + CT arm versus the CT arm. Updated OS analyses in the bTMB ≥20 and <20 mut/Mb subgroups, after median follow-up of >5 years (data cut-off 24 August 2023), were similar to those obtained at the primary OS data cut-off. CONCLUSIONS: First-line treatment with T (limited course) plus D (until progression) and four cycles of CT consistently improved clinical outcomes versus CT alone in both bTMB-high and -low subgroups, and also in both high and low tTMB subgroups, in patients with mNSCLC. Benefit appeared greater in the TMB-high versus TMB-low subgroups; the addition of anti-cytotoxic T lymphocyte-associated antigen-4 to anti-PD-L1 and CT seemed to increase the magnitude of this difference.