Background: The characteristics of esophagogastric junction adenocarcinoma (EGJA) differ from those of gastric adenocarcinoma (GA). Although chemotherapy for advanced GA has been adapted for advanced EGJA, its efficacy remains unknown. This study aims to elucidate the effectiveness of chemotherapy for advanced EGJA in real-world practice. Methods: This retrospective study compared treatment outcomes between advanced GA and EGJA in patients with performance status (PS) 0 to 2 who received doublet or triplet chemotherapy, including platinum agents, as first-line treatment at our hospital between 2010 and 2020. Patients with non-curative resection factors limited to positive peritoneal cytology, or those who relapsed during adjuvant chemotherapy or within 6 months after its completion, were excluded. Results: Out of the total 825 patients, 765 had GA and 60 had EGJA. The baseline characteristics of the GA and EGJA groups were as follows, respectively: median age 65.0 and 59.5 years (p = 0.02); PS 0, 35.9% and 33.3%; PS 1, 57.5% and 65.0%; PS 2, 6.5% and 1.7% (p = 0.28); HER2 positive, 16.1% and 26.7% (p = 0.06); diffuse histology, 52.7% and 26.7% (p < 0.01); lymph node metastasis, 74.0% and 93.3% (p < 0.01); peritoneal metastasis, 66.4% and 25.0% (p < 0.01); liver metastasis, 26.4% and 40.0% (p = 0.03); lung metastasis, 6.5% and 16.7% (p < 0.01). The proportions of fluoropyrimidine, taxanes, and irinotecan in all treatment lines did not differ between the two groups (fluoropyrimidine, 99.7% vs. 100%; taxanes, 70.1% vs. 73.3%; irinotecan, 23.0% vs. 23.3%). However, the EGJA group used ramucirumab, trastuzumab, and immune checkpoint inhibitors (ICIs) significantly more frequently than the GA group (ramucirumab, 27.8% vs. 43.3%, p = 0.02; trastuzumab, 15.4% vs. 26.7%, p = 0.03; ICIs, 21.0% vs. 41.7%, p < 0.01). Overall survival did not differ between the two groups (median, 14.8 months vs. 18.5 months, HR 0.80; 95% CI, 0.59-1.10, p = 0.17). Moreover, multivariate analysis revealed that EGJA and GA had a comparable prognosis (HR 1.23; 95% CI, 0.87-1.72, p = 0.24). HER2 positivity was a favorable prognostic factor in multivariate analysis, whereas diffuse histology, poor PS, lymph node metastasis, liver metastasis, and the presence of peritoneal metastasis were poor prognostic factors. There was no difference in time-to-treatment failure between the two groups for first-line (median, 6.1 vs. 4.9 months, HR 0.96; 95% CI, 0.73-1.28, p = 0.80), second-line (median, 2.9 vs. 3.5 months, HR 0.99; 95% CI, 0.73-1.34, p = 0.93), and third-line treatment (median, 1.9 vs. 2.7 months, HR 0.78; 95% CI, 0.53-1.13, p = 0.62). Conclusions: In real-world practice, the efficacy of chemotherapy for advanced EGJA was comparable to that for advanced GA.