Objective <p>Ovarian cancer remains the most lethal gynaecological malignancy, and the identification of reliable biomarkers for determining treatment responses and prognosis is needed. This study investigated the clinical significance of helper and cytotoxic effector memory T-cell (T_EM) subsets, defined by CD27 and CD28 coexpression, circulation and local compartments.</p> Methods <p>Multi-parametric flow cytometry was used to analyse peripheral blood (<i>n</i> = 30), ascites (<i>n</i> = 14) and tumour tissue (<i>n</i> = 31) from patients with high-grade ovarian cancer (FIGO III-C, <i>n</i> = 31) and age-matched healthy controls (<i>n</i> = 16). Subsets were stratified by histopathology, neoadjuvant chemotherapy status, and clinical outcomes.</p> Results <p>T_EM cells were identified as the predominant T-cell population across all the compartments, with significant enrichment within the tumour microenvironment (reaching &gt; 90%). In peripheral blood from healthy controls, T_EM cells likewise represented the dominant memory T-cell subset, with CD27⁺CD28⁺ cells constituting the most abundant T_EM cell fraction. The CD27⁺CD28⁺ T_EM cell subset emerged as the unequivocally dominant T_EM fraction in both circulation and tumour tissue. Notably, a high frequency (&gt; 63%) of circulating CD27⁺CD28⁺ helper or cytotoxic T_EM cells significantly correlated with prolonged progression-free survival (15–16 months vs. 9–10.5 months), an increased incidence of reactive lymph nodes, and favourable CA125 kinetics. Conversely, neoadjuvant chemotherapy was associated with a significant reduction in tumour-infiltrating CD27⁺CD28⁺ T_EM cells.</p> Conclusion <p>The preservation of CD27 and CD28 on circulating T_EM cells was associated with favourable clinical outcomes and markers of active immunosurveillance, whereas the loss of these molecules may indicate terminal differentiation and impaired antitumour immunity in patients with ovarian cancer.</p>