Background: We investigated the incidence of reactive thrombocytosis after maximal cytoreductive surgery in advanced epithelial ovarian cancer and its role on survival. Methods: We retrospectively reviewed electronic medical records of patients who underwent primary cytoreductive surgery for advanced epithelial ovarian/fallopian cancer or peritoneal cancer from January 1, 2012, and December 31, 2017. In addition to known clinical prognostic factors, we analyzed the correlation serum platelet counts and prognosis at various time points including before surgery, during peri-operative period, and on each cycle of adjuvant chemotherapy.Results: 474 patients were eligible for the analysis. Median age was 54 years (18-88). 401 patients (84.6%) were FIGO stage III and 405 patients (85.4%) were serous adenocarcinoma. 79 patients (22.6%) had splenectomy and optimal cytoreduction (residual < 1 cm) was achieved at 326 patients (68.8%). A week after surgery, thrombocytosis was observed in 229 patients (48.3%) patients in the entire cohort. Especially, higher platelet counts were observed in patients with splenectomy compared with patients without splenectomy at various time points after surgery. Subgroup of patients who had persistent thrombocytosis during adjuvant chemotherapy showed poor survivals. In particular, thrombocytosis on 5th cycle of adjuvant chemotherapy showed most significant impact on overall survival (HR; 1.871, 95%CI; 1.034-3.386, p = 0.038) among various time points in multivariate analysis. In a logistic regression model, splenectomy (p < 0.001) significantly attributed to thrombocytosis on 5th cycle. Conclusion: Reactive thrombocytosis after primary cytoreductive surgery is associated with poor survival in advanced epithelial ovarian cancer, particularly when thrombocytosis was observed during adjuvant chemotherapy.