Background: ATTRACTION-2 phase III trial proved the clinical efficacy of nivolumab (Nivo) in AGC patients (pts) treated with ≥ 2 previous chemotherapy regimens. However, the benefits of Nivo seem to be limited to a subset of pts and there is a need to identify predictive markers to select pts who would benefit from Nivo. Methods: Clinical data and tumor samples of AGC pts enrolled from Asan Medical Center in ATTRACTION-2 study were retrospectively analyzed. PD-L1 (+) was defined as combined positive score of ≥ 1%. EBV and MSI status were determined by EBV-encoded small RNA in situ hybridization and IHC for MLH-1, MSH-2, PMS-2, and MSH-6, respectively. Tumor mutation burden (TMB) was acquired by targeted next-generation sequencing using the NextSeq platform with OncoPanel. Results: A total of 45 pts (28/17 in Nivo/placebo arms) were eligible for the analysis. Baseline neutrophil-lymphocyte ratio (NLR) was median 2.9 and 7 pts had hyponatremia ( < 135). In 36 pts with available tissues, there were PD-L1 (+) (N = 13, 36.1%), EBV (+) (N = 6, 16.7%), and no MSI-high. TMB data could be acquired in 29 pts and median TMB was 8.2/Mb (0.0-21.3). With a median follow-up of 28.3 months (mo) in surviving pts, objective response rate, median progression-free survival (PFS), and overall survival (OS) were 16.7%, 1.6 mo, and 8.1 mo in Nivo arm and 0%, 1.6 mo and 6.5 mo in placebo arm. In Nivo arm with measurable lesions, PD-L1 (+) pts had significantly higher disease control rate than PD-L1 (-) pts (87.5% vs. 20%, p = 0.015). In multivariate model adjusted for important factors (age, sex, Nivo vs. placebo, treatment line, NLR, Na, and PD-L1), NLR ≤ 2.9 and PD-L1 (+) were significant factors for PFS (Hazard Ratio [HR] 0.47, p = 0.037 and HR 0.32, p = 0.006, respectively) and PD-L1 (+) was a significant factor for OS (HR, 0.44, p = 0.012). With adjusting these factors with TMB, PD-L1 (+) remained favorable for PFS and OS. Pts with NLR ≤ 2.9 or PD-L1 (+) or Na > 135 significantly favored Nivo compared to placebo in terms of PFS and OS. Conclusions: Baseline NLR and PD-L1 status may be relevant predictive markers to select pts with AGC who would benefit from Nivo.