Introduction: Health literacy is a key determinant of diabetes self-management; however, the behavioral mechanisms linking health literacy to health maintenance efficacy in adults with type 1 diabetes mellitus (T1DM) remain insufficiently explored. Health maintenance efficacy refers to individuals’ perceived ability to sustain long-term diabetes care behaviors. Methods: This cross-sectional analytical study was conducted with 368 adults diagnosed with T1DM in Turkey. Data were collected online using the European Health Literacy Survey Questionnaire (HLS-EU-Q6) and the treatment adherence and health maintenance efficacy subscales of the Chronic Illness Self-Management Scale. Descriptive statistics, Pearson correlation analysis, and mediation analysis were performed using Hayes’ PROCESS Model 4 with 5000 bootstrap resamples. Results: Health literacy was positively associated with both treatment adherence and health maintenance efficacy. Treatment adherence played a partial mediating role in the association between health literacy and health maintenance efficacy (indirect effect = 0.11; 95% CI: 0.06–0.17). The final model explained 38% of the variance in health maintenance efficacy. Conclusion: Health literacy was strongly associated with health maintenance efficacy in adults with T1DM, both directly and indirectly through its association with treatment adherence. By conceptualizing health maintenance efficacy as a distinct construct reflecting the sustainability of diabetes care behaviors, the findings provide clearer insight into how health literacy relates to long-term self-management capacity. Given the cross-sectional design, these relationships should be interpreted as associative rather than causal.