Gastrointestinal (GI) cancers present significant health challenges, necessitating strategies to improve patients' health-related quality of life (HRQoL). Health literacy (HL) and patient activation (PA) are key factors in patient self-management, yet their interplay and impact on HRQoL remain unclear. This study investigates the relationship between HL, PA, and HRQoL in GI cancer patients, with a focus on PA as a potential mediator between HL and HRQoL. We conducted a cross-sectional analysis using baseline data from the multicenter OptiScreen study. HL, PA, and HRQoL were assessed using validated instruments: the European Health Literacy Survey Questionnaire (HLS-EU-Q16), the Patient Activation Measure (PAM-13D), and the Short-Form Health Survey (SF-8). Statistical analyses included correlation tests and mediation modeling. Out of 854 eligible GI cancer patients, 397 (response rate = 46%) participated in the study. HL was positively correlated with PA, r(359) = 0.37, p <
.002 and with physical and mental HRQoL, r(322) = 0.12, p = .035 and r(322) = 0.20, p <
.002, respectively. We found that the relationship between HL and mental HRQoL is fully mediated by PA (indirect effect: 0.186, 95% CI [0.016, 0.385]). Our findings highlight the crucial role of PA in enhancing mental HRQoL in GI cancer patients, suggesting that interventions targeting both HL and PA could improve patient outcomes. Future research should explore phase-specific interventions and broader psychological factors affecting patient self-management and well-being.