Background: CapeOX is a standard regimen for treating advanced gastric cancer (AGC). However, limited data are available for its use in the elderly patients. Geriatric assessment (GA) assessment is also considered important other than the age. Therefore, we conducted a multicenter, single-arm, phase II study to evaluate the efficacy and safety of CapeOX as a first-line therapy for AGC in elderly patients. We performed GA assessments using the G8, and the MINI-COG. Methods: The inclusion criteria were: HER2-negative or unknown status, age 70 years or older, PS 0 or 1, no history of chemotherapy or radiotherapy, and an evaluable lesion based on the RECIST v1.1 guidelines. The initial treatment plan consisted of capecitabine (1,000 mg/m², b.i.d., on days 1–14), and oxaliplatin (130 mg/m², on day 1) every three weeks. After the initial feasibility assessment, the treatment plan was modified and consisted of capecitabine (750 mg/m², b.i.d., on days 1–14) and oxaliplatin (100 mg/m², on day 1) every three weeks. The primary endpoint was overall survival (OS). G8 assessments were performed before the beginning of the treatment, every three months after the start of the treatment, and at the end of the treatment. MINI-COG assessments were performed before the beginning of the treatment, and at the end of the treatment. Results: Between November 2016 and June 2020, 108 patients were enrolled, out of which 104 were evaluated. Thirty-nine patients received the initial treatment whereas sixty-five patients received the modified treatment. The primary endpoint was met. The median OS, progression-free survival (PFS), and time-to-treatment failure (TTF) were 12.9 (95% CI: 11.7–14.8), 5.8 (95% CI: 5.1–7.1), and 4.3 (95% CI: 3.9–5.7) months, respectively, for all patients. The median OS, PFS, and TTF for the initial treatment plan were 13.4 (95% CI: 9.6–16.0), 5.8 (95% CI: 4.1–7.9), and 5.3 (95% CI: 3.5–7.3) months, respectively, and for the modified treatment plan were 12.8 (95% CI: 11.3–15.3), 5.7 (95% CI: 4.4–7.0), and 4.2 (95% CI: 3.7–5.7) months, respectively. The most common grade 3/4 toxicities in the initial treatment plan patients were neutropenia (17.9%), anemia (12.8%), thrombocytopenia (12.8%), anorexia (10.3%), and nausea (10.3%) and in the modified treatment plan patients were (13.8%) and anorexia (12.3%). Mean G8 score was 11.4 before the treatment, 13.4 during the treatment and 11.6 at the end of the treatment. Mean MINI-COG score was 4.0 before the treatment, and 4.1 at the end of the treatment and the positive screen for dementia was 13.5% before the treatment and 11.4% at the end of the treatment. Conclusions: Our results show that the modified treatment plan is favorable for the elderly patients with gastric cancer based on the OS and the toxicities. Clinical trial information: 000022450.