Health literacy (HL) is a crucial factor influencing health-related decisions, including dietary patterns. The Mediterranean diet is widely recognized as one of the healthiest eating patterns. This study aims to explore the association between HL levels among adults in socially vulnerable contexts and their adoption of the Mediterranean diet. Conducted as part of a cluster-randomized trial in primary schools, the research involved 557 parents of children aged 6 to 10 who consented to participate. HL was assessed using the HLS questionnaire, while adoption of the Mediterranean diet was evaluated using the MEDAS questionnaire. Participants also provided anthropometric and sociodemographic data via a questionnaire, from which BMI was calculated (weight/height2). Education level was categorized as higher education or lower and professional status was classified based on occupational categories. Generalized linear models and logistic regression were employed for analysis. The findings indicate a direct association between HL level and adoption of the Mediterranean diet (B = 0.022, 95% CI 0.010–0.035, p < 0.001), underscoring the influence of HL on dietary choices. Specifically, the healthcare dimension of HL emerges as pivotal in shaping dietary behaviors, particularly towards the Mediterranean diet. These results underscore the importance of policies and programs aimed at enhancing HL, especially among socially vulnerable populations. Prioritizing public health interventions to improve HL is critical for promoting better dietary decision-making.