Background: NIVO therapy is a standard care treatment for heavily pretreated patients with AGC. Improvement in objective response rate (ORR) to CTx after NIVO therapy for various cancer types has been reported. However, the efficacy and safety of CTx for AGC after progression on NIVO remains unclear. Methods: The REVIVE trial was a prospective, multicenter, observational study that evaluated the efficacy and safety of CTx in NIVO-refractory or NIVO-intolerant patients (pts) with AGC (UMIN000032182). The primary endpoint was overall survival (OS) of CTx following NIVO therapy. The median threshold and expected survival times were set as 4.0 and 7.0 months (M). The secondary endpoints are ORR, disease control rate (DCR), progression-free survival (PFS), and incidence of adverse events (AEs), including immune-related adverse events (irAEs). CTx consisted of irinotecan alone (IRI), trifluridine/tipiracil alone (FT), and oxaliplatin-containing regimens (OX). Results: Of 395 pts treated with NIVO who met the eligibility at formal registration from Jun 2018 to Sep 2020, 199 pts who received CTx after NIVO were evaluated. Pt characteristics were as follows: median age, 69 years; male, 70%; ECOG PS 0/1/2, 38/51/12%; histology (diffuse/intestinal), 39/59%; metastatic lesions (peritoneum/liver/lung), 38/34/15%; number of metastatic organ sites (0–1/≥2), 40/60%; measurable lesions, 83%; and CTx regimens (IRI/FT/OX), 64/31/5%. Median OS and PFS were 7.5 M (95%CI, 6.7–9.7) at 145 events for OS and 2.9 M (95%CI, 2.2–3.5) at 184 events for PFS. The ORR and DCR were 17.0% (95%CI, 11.6–23.6) and 46.7% (95%CI, 38.9–54.6). Median OS, median PFS, ORR, and DCR according to CTx regimens (IRI/FT/OX) were 8.1/7.1/6.2 M, 3.3/2.8/2.4 M, 18.9/10.9/25.0%, and 47.8/43.5/50.0%, respectively. At the start of CTx, 42 pts had irAEs due to prior NIVO therapy. The most common any-grade and grade ≥3 AEs during CTx included decreased appetite (46% and 7.5%), fatigue (26% and 2.5%), nausea (24% and 1.5%), constipation (16% and 0%), and diarrhea (28% and 4.0%). No treatment-related deaths were observed. Conclusions: Prior NIVO therapy may lead to improved prognosis after CTx without unexpected AEs in pts with AGC, warranting further investigations after NIVO is approved as first-line treatment.