Introduction: The peritoneum is a predilection site for gastric cancer metastases. Current standard treatment for gastric cancer patients with synchronous peritoneal metastases is palliative systemic therapy. However, its efficacy is largely unknown. The aim of this study was to investigate the incidence, treatment and survival patterns of gastric cancer patients with synchronous peritoneal metastases in the Netherlands. Methods: All newly diagnosed gastric adenocarcinoma patients with synchronous peritoneal metastases between 1999 and 2017 were selected from the Netherlands Cancer Registry (NCR). Incidence, treatment and survival patterns were analyzed. Results: In total, 3,773 patients were identified from the NCR. The incidence of synchronous peritoneal metastases in gastric cancer patients increased from 18% in 2008 to 27% in 2017. The use of systemic therapy increased from 15% in 1999–2002 to 43% in 2013–2017 (p < 0.001). The median survival of the entire cohort did not significantly increase over time. Median survival of patients treated with systemic therapy increased from 7.4 months in 1999–2002 to 9.4 months in 2013–2017 (p = 0.005). In contrast, median survival of patients not treated with systemic therapy decreased from 3.3 months in 1999–2002 to 2.1 months in 2013–2017 (p < 0.001). Some clinical and pathological data such as the extent of the peritoneal metastases were not available. Conclusion: Synchronous peritoneal metastases are increasingly diagnosed in gastric cancer patients. In recent years, more patients were treated with systemic treatment and survival of these patients increased. However, as survival of the entire group did not improve over time, the effect of systemic therapy remains unknown.