Background It would be useful to have criteria for predicting long-term treatment responses to immune checkpoint inhibitors (ICIs). Maximum depth of response correlates with treatment outcomes among patients receiving programmed death protein 1 axis inhibitors for non-small cell lung cancer (NSCLC). We investigated associations between early depth of response and survival outcomes among patients receiving nivolumab for NSCLC. Methods Using records from prospective observational cohorts, we identified 83 previously treated advanced patients with NSCLC who received nivolumab during 2016-2017. Thirty-one patients who achieved disease control were analyzed. Tumor assessments followed the Response Evaluation Criteria in Solid Tumors (RECIST). Using Kaplan-Meier and receiver operating characteristic (ROC) curve analyses, treatment outcomes were compared with percent tumor reductions from baseline to the first evaluation (8-12 weeks after starting nivolumab). Results Early depth of response was predictive of 6-month progression-free survival (area under the ROC curve, 0.848). Based on ROC results, early tumor shrinkage was defined as a > 10% reduction by the first evaluation. Early tumor shrinkage was associated with significantly longer median progression-free survival (early tumor shrinkage: 16.6 months, 95% confidence interval [CI] 8.5 months-not reached; no early shrinkage: 5.1 months, 95% CI 3.9-6.8 months; P < 0.001) and significantly longer median overall survival (P = 0.046). Conclusions Early depth of tumor shrinkage was associated with outcomes after ICI treatment. Because of its simplicity and predictive ability, early tumor shrinkage may be a promising factor for use in clinical settings. However, confirmation of our results is needed.