Background: Immune checkpoint inhibitor (ICI) or irinotecan-based chemotherapy are frequently used after paclitaxel plus ramucirumab as second-line treatment for patients with recurrent and/or metastatic gastric cancer (RMGC). This study aimed to compare the efficacy between ICI and irinotecan-based chemotherapy as third-line treatment in patients with RMGC. Methods: We retrospectively reviewed patients with RMGC, whose third-line treatment started between July 2019 and June 2021 at 17 centers in South Korea. The ICI group included patients who received nivolumab or pembrolizumab and the irinotecan-based chemotherapy group included those patients who received irinotecan or FOLFIRI. Results: A total of 363 patients (129 in the ICI group and 234 in the irinotecan-based chemotherapy group) were analyzed. The median progression-free survival (PFS) was 2.2 and 2.9 months in the ICI and irinotecan-based chemotherapy groups, respectively. The median overall survival (OS) was 5.5 months (95% CI, 3.6-7.4) in the ICI group and 6.0 months (95% CI, 4.8-7.0) in the irinotecan-based chemotherapy group [HR 0.97 (95% CI, 0.75-1.25); P=0.786]. Multivariable Cox-regression analysis showed that weight loss, peritoneal metastasis, low serum sodium, low serum albumin and short duration of second-line treatment were associated with inferior OS (P<0.05), meanwhile microsatellite instability-high (MSI-H)/mismatch repair-deficient (MMR-D) tumor was an independent prognostic factor for superior OS. The ICI group showed significantly longer OS than the irinotecan-based chemotherapy group in patients without peritoneal metastasis [HR 0.54 (95% CI 0.30-0.99); P=0.047)]. Whereas the irinotecan-based chemotherapy group showed significantly longer OS in patients without PD-L1 expression [HR 1.62 (95% CI 1.03-2.55); P=0.037] than the ICI group. Conclusions: No significant difference in survival outcome was observed between ICI and irinotecan-based chemotherapy as third-line treatment for patients with RMGC. ICI might be preferred for patients without peritoneal metastasis and irinotecan-based chemotherapy for patients with tumors without PD-L1 expression.