Purpose <p>The role of conversion surgery for gastric cancer with peritoneal metastasis is unclear and there are no reliable biomarkers. We evaluated the outcomes of conversion surgery and the predictive value of inflammatory and nutritional markers.</p> Methods <p>The subjects of this retrospective analysis were 65 patients with massive peritoneal metastasis (P1b or P1c) from among 102 gastric cancer patients with positive peritoneal cytology or peritoneal metastasis, diagnosed by staging laparoscopy. We investigated the association between inflammatory and nutritional markers before treatment, including the neutrophil-to-lymphocyte ratio, the modified Glasgow prognostic score (mGPS), the prognostic nutritional index; and the feasibility of conversion surgery.</p> Results <p>Conversion surgery was performed after chemotherapy in 14 of the 65 patients. Median survival was longer in the conversion surgery group than in the non-surgery group (30 vs. 9 months, <i>P</i> &lt; 0.001). Multivariate analysis identified conversion surgery (HR 0.27, 95% CI 0.12–0.60) and mGPS 0 (HR 0.35, 95% CI 0.19–0.65) as independent prognostic factors. Patients with mGPS 0 were significantly more likely to undergo conversion surgery (<i>P</i> = 0.026).</p> Conclusions <p>Conversion surgery may be associated with improved survival for gastric cancer patients with unresectable peritoneal metastases. Moreover, mGPS may be a useful biomarker as a prognostic or selection-related factor.</p>