OBJECTIVE: The aim of this study was to analyse whether the association between adherence to the Mediterranean diet and health-related quality of life (HRQoL) is mediated by physical fitness components (i.e., cardiorespiratory fitness, lower-limb strength, upper-limb strength and motor competence) in adolescents. METHODS: This is a cross-sectional study including mediation analyses. This study involved 181 adolescents (86 girls) aged 15.8 ± 0.3 years recruited from secondary schools and sport clubs located in Castellon (Spain). Adherence to the Mediterranean diet was evaluated using the KIDMED questionnaire. HRQoL was assessed with the KIDSCREEN-10 questionnaire. Cardiorespiratory fitness was assessed using the 20-m shuttle run test. Lower-limb strength was assessed through the standing broad jump test. Upper-limb strength was evaluated using a handgrip dynamometer. Motor competence was assessed using the 4 × 10 m shuttle run test. Mediation analyses were performed for each physical fitness component in order to assess its mediating effect on the association between adherence to the Mediterranean diet and HRQoL. Indirect effects (IE) with confidence intervals (CI) not including zero were interpreted as statistically significant. RESULTS: Cardiorespiratory fitness and lower-limb strength had a mediating effect on the positive association between adherence to the Mediterranean diet and HRQoL (IE = 0.11, 95 % CI = 0.01; 0.27 and IE = 0.07, 95 % CI = 0.01; 0.19). Upper-limb strength and motor competence did not act as mediators in the association between adherence to the Mediterranean diet and HRQoL (IE = 0.00, 95 % CI = -0.04; 0.07 and IE = 0.01, 95 % CI = -0.03; 0.07, respectively). CONCLUSION: Our findings showed that the positive association of adherence to the Mediterranean diet on adolescents' HRQoL was mediated by physical fitness. Educational and public health strategies aiming to improve Spanish adolescents' HRQoL should focus on diet and physical fitness.