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Summary: Background: The peritoneum is a common metastatic site in gastric cancer. The prognosis of synchronous peritoneal metastases compared to other metastatic sites in gastric cancer remains understudied. This study aims to evaluate the impact of peritoneal metastases on survival in patients with metastatic gastric cancer. Methods: Patients with gastric cancer and synchronous metastases between 2015 and 2020 were identified from the nationwide Netherlands Cancer Registry. Patients were categorized based on the site of metastases. Median overall survival (OS) was calculated for each metastatic site group. Multivariable Cox regression analyses were performed to evaluate the association between patient, tumour, and treatment characteristics, including the impact of systemic therapy, on OS. Findings: A total of 4072 patients were included, of whom 1835 (45.1%) had peritoneal metastases. Of these, 58.1% had isolated peritoneal metastases. For patients with metastatic gastric cancer treated with systemic therapy, the median OS was 9.0 months (95% confidence interval (CI): 8.6–9.5), compared to 1.7 months (95% CI: 1.7–1.9) for treatment-naïve patients, who received only palliative care. The survival for patients with isolated peritoneal metastases (4.4 months, 95% CI: 4.0–4.8 months) was similar to those with isolated non-peritoneal metastases (4.6 months, 95% CI: 4.2–5.1 months, adjusted HR: 0.94, 95% CI: 0.86–1.03, p = 0.185). Systemic therapy was associated with comparable survival in patients with peritoneal metastases and those with metastases at other sites. Interpretation: This study demonstrates that there is no statistically significant difference in survival between patients with isolated peritoneal metastases and those with isolated non-peritoneal metastases in gastric cancer. Our findings emphasize the unique prognostic landscape for peritoneal metastases in gastric cancer, underscoring the need for disease-specific evaluations, rather than relying on assumptions derived from other cancer types. Funding: None.