Objective: Malnutrition is frequent in ovarian cancer (OC) patients and may compromise post-operative outcomes. The aim of this study is to evaluate the impact of pre-operative immunonutrition on the surgical outcome of OC patients, and on their nutritional, inflammatory and peripheral blood immune status. Methods: A prospective study was performed between September 2016 and April 2020. Immune-enhancing enteral nutrition was administered to 42 patients before surgery according to their nutritional status assessed by the Malnutritional Universal Screening Tool. Biochemical and hematological monitoring was performed before and after immunonutrition. Post-operative outcomes were assessed and compared with those of a similar group of patients treated without nutritional support. Results: Of the 42 immune-nourished patients, 23 (54.8%) had a low, 11 (26.2%) an intermediate and 8 (19%) a high risk of malnutrition. After the immunonutritional intake, significant variations of prealbumin, creatinine and white blood cells were detected. All T cell populations had an increasing trend, in particular CD3+ T lymphocytes (p=0.020), CD3+CD8+ cytotoxic T lymphocytes (p=0.046) and lymphocyte with HLA-DR expression (p=0.012). The rate of grade II–III post-operative complications was lower (21.4% vs. 42.9%, p=0.035) and the time of hospitalization was shorter (7.5 vs. 9.2, p=0.009) in the immune-nourished group. Conclusion Pre-operative immunonutrition improves the surgical outcome of OC patients. After immunonutrition, an increase of CD3+CD8+ cytotoxic T lymphocytes was observed.