ABSTRACT Objective To analyze the effects of educational intervention on health literacy and knowledge about diabetes in adults assisted in primary health care. Method This is a quasi-experimental study with educational intervention during the nursing consultation with 33 adults diagnosed with type 2 diabetes mellitus registered in a Family Health Strategy Unit in southern Brazil and involving group activity and telephone follow-up. Sociodemographic and clinical instruments were applied before and after the intervention; these instruments included the Spoken Knowledge in Low Literacy Patients with Diabetes and the Eight-Item Health Literacy Assessment Tool. Data were analyzed using the Wilcoxon, Spearman, and McNemar correlation coefficient tests. Results Most participants were women (69.7%), with a mean age of 57.0 years, less than nine years of schooling (69.7%), and ≤10 years of diagnosis (51.5%). After the intervention, there was an increase in knowledge about diabetes (p = 0.001), correlated with health literacy (r = 0.494; p = 0.001) and the time of diagnosis (r = 0.455; p = 0.001). Conclusion and implications for practice The instruments to measure health literacy and knowledge about diabetes enabled the construction of educational strategies aimed at existing gaps, increasing the knowledge, thereby favoring the development of skills for self-management.