Background: The gut microbiome (GM) plays an important role in shaping systemic immune responses. Preclinical and clinical data suggest that GM influences anti-PD-1/PD-L1 or -CTLA-4 Antibody (Ab)-mediated anti-cancer responses. Furthermore, there is strong evidence that antibiotics (ATB) worsen clinical outcomes based on multiple retrospective and one prospective studies using immune checkpoint inhibitor (ICI). However, whether GM profiling, at baseline or post-ATB, could represent a biomarker of response in advanced non-small cell lung cancer (NSCLC) during ICI therapy remains unknown. Methods: We prospectively collected baseline (pre-ICI) fecal samples and clinical data Japanese patients (pts) with NSCLC treated with anti-PD-1/PD-L1 Abs in first or second-line therapy. We performed a 16S rRNA V3-V4 sequencing of gene amplicons of fecal microbes. Amplicon sequence variants were generated with dada2 R package. Diversity analysis was performed with phyloseq R. Differential abundance analysis was performed with both LEfSe and DESeq2 methods. Clinical endpoints were progression-free survival (PFS), overall survival (OS), objective response rate (ORR), and immune-related adverse events (irAE). Results: 70 fecal samples were analyzed. Median OS and PFS in all patients were 16.1 and 5.2 months, respectively. 16 pts (23%) were exposed to ATB 1 month prior to ICI initiation. Pts on ATB had lower α-diversity at baseline and underrepresentation of Clostridiales and Ruminococcaceae UCG 13. When analyzing ATB-free pts, lower α-diversity was observed in non-responders. In addition, Ruminococcaceae UCG 13 was enriched in patients with OS > 12 months, favorable ORR, and PFS > 6 months. Clostridiales order was also enriched in patients with OS > 12 months. Compositional GM differences were also observed between the patients who experienced clinically significant (≥grade 2) irAE; Lactobacillaceae and Raoultella were enriched in pts who had no significant irAE. Conclusions: We demonstrated the negative influence of ATB on GM composition and identified differential bacteria repertoire in pts experiencing favorable clinical outcomes or low grade irAE. Our data pave the way to the development of diagnosis tools aimed at identifying gut dysbiosis to predict resistance or irAE during ICI for NSCLC.