Cancer immunotherapy, including atezolizumab monotherapy, is a promising alternative strategy for patients with advanced non-small-cell lung cancer (NSCLC). Several inflammatory indices have been reported as potential biomarkers regarding the effectiveness of various treatments. This study aimed to analyze the efficacy of atezolizumab monotherapy using baseline inflammatory markers in NSCLC patients. We retrospectively enrolled 81 NSCLC patients who received atezolizumab monotherapy at six different medical institutions in Japan. The Cox proportional hazards model was used to assess the impact of the clinical variables, including inflammatory indexes, on clinical outcomes. Median progression-free survival (PFS) and overall survival (OS) were 60 days and 252 days, respectively. The objective response rate was 7.4%, and the disease control rate was 54.3%. Patients with high neutrophil to lymphocyte ratio (NLR), low lymphocyte to monocyte ratio (LMR), and/or high platelet to lymphocyte ratio (PLR), at baseline, demonstrated substantially shorter PFS and OS compared to those with a low NLR, high LMR, and/or low PLR. The multivariate analysis demonstrated that a high baseline NLR was substantially associated with short PFS and short OS. Our retrospective observations suggest that inflammatory indices may be a potential negative prognostic factor of atezolizumab monotherapy outcomes in NSCLC patients.