Background The clinical features and therapeutic strategies for gastric cancer with positive peritoneal washing cytology but without visible gross peritoneal metastasis have not been defined. The aim of this study was to evaluate the effect and clinical prognostic value of postoperative chemotherapy in gastric cancer patients with positive peritoneal washing cytology without gross peritoneal metastasis who underwent radical D2 gastrectomy in terms of disease-free survival (DFS) and overall survival (OS). Materials and Methods Intraoperative peritoneal washing cytology was performed in 285 patients who underwent radical D2 gastrectomy between April 2004 and May 2016. Of them, 88 patients with positive cytology but without gross peritoneal metastasis were included in the study. In total, 64 patients received postoperative chemotherapy, whereas 24 patients underwent surgery only. Results Most gastric cancer patients with positive cytology without gross peritoneal metastasis demonstrated pT4 and/or pN3 disease. Postoperative chemotherapy improved DFS and OS compared to surgery only in gastric cancer patients with positive cytology without gross peritoneal metastasis (median DFS 11.63 vs. 6.98 months, p < 0.001; median OS 25.50 vs. 12.11 months, p < 0.001). In multivariate analyses of gastric cancer patients with positive cytology without gross peritoneal metastasis, no chemotherapy was the strongest clinical factor for poorer DFS (hazard ratio [HR] 3.76, p < 0.001) or OS (HR 4.37, p < 0.001). Conclusion Postoperative chemotherapy improves the survival outcome compared to surgery alone in gastric cancer patients with positive peritoneal washing cytology but without visible gross peritoneal metastasis who underwent radical D2 gastrectomy.