Introduction <p>Medication adherence is crucial for effective blood sugar control in diabetes patients. However, older adults face a higher risk of non-adherence. This study aims to evaluate the effectiveness of a theory-based program designed to enhance medication adherence among older adults with type 2 diabetes.</p> Methods <p>This quasi-experimental study involved 100 elderly individuals with type 2 diabetes in Kermanshah, Iran, in 2024. Participants were randomly assigned to an intervention (50) or control group (50). The intervention group attended six 40-minute sessions over six weeks, based on the Information-Motivation-Behavioral Skills (IMB) model. Data were collected via a questionnaire assessing demographics, IMB model components, and medication adherence. Data were analyzed using SPSS version 16, employing chi-square tests and t-tests for comparisons, moreover, effect sizes were determined using Cohen’s d.</p> Results <p>The intervention group showed significant improvements, especially in “Information,” with a very large effect size of 1.3 and an average change of 3.84 (<i>p</i> &lt; 0.001). In contrast, the control group had minimal changes. Other components like “Attitude” and “Self-efficacy” also had large effect sizes (0.65 and 0.67). However, “Subjective Norms” showed a small effect size (0.11) with no significant changes. Additionally, the intervention group showed a significant improvement in medication adherence (effect size 1.15), increasing scores from 16.92 to 19.76, (<i>p</i> &lt; 0.001) while the control group had minimal changes.</p> Conclusion <p>The program based on the IMB model significantly improved medication adherence among elderly individuals with type 2 diabetes. We also identified the key theoretical methods of behavior change for each determinant of the IMB model and recommend utilizing them in the development of similar future interventions.</p>