Health literacy (HL) assumes individuals' knowledge, motivation, and competencies to access, understand, evaluate, and apply health information to make judgments and decisions in daily life, related to healthcare, prevention of diseases, and health promotion to maintain or improve quality of life. The study aimed to measure the level of HL and associated factors: sex, age, schooling, income, skin color, self-rated health status, type of diabetes, and presence of comorbidities. The authors assessed 107 adults with diabetes followed at a public outpatient clinic in the city of Rio de Janeiro, Brazil. The sample did not include first-time patients, patients without a diagnosis of diabetes, or with visual or hearing impairment. HL was assessed with the Brazilian version of the European Health Literacy Survey (HLS-EU-BR). Simple and multiple ordinal logistic regression models were constructed, considering four levels of HL as the dependent variables. The associations were expressed as odds ratios (OR). Approximately 95% of the sample showed poor or limited HL (94.8%; 95%CI: 90.3-99.3). Female gender, older age, and lower schooling were associated with lower odds of excellent HL. In the adjusted model, only schooling remained statistically significant in its effect on HL (adjusted OR = 0.41; 95%CI: 0.17-0.98; p < 0.05). Schooling was the characteristic most strongly related to level of HL.