Diabetes mellitus is a health complication that millions of people suffer from all over the world. Type II (non-insulin dependent) diabetes requires many changes in the daily lives of patients, including monitoring blood glucose, following a healthy diet, exercising, and taking medications. Although it is vital for their health, patients generally find it difficult to adhere to their medical regimen. In order to better understand the adherence behaviors of type II diabetes patients, the theory of planned behavior (TPB) was used as the theoretical framework for this study. Ninety type II diabetes patients, who were outpatients of four different hospitals in Ankara, Turkey were administered the TPB tool. The mediation analyses provided support for the mediating role of intention for the attitudes-behavior and subjective norms-behavior relations. The findings did not reveal a mediating role of intention for the PBC-behavior relation but a significant direct effect of PBC on adherence behavior was found. Overall, it seems important that PBC and the multi-faceted nature of adherence behaviors are considered when designing interventions for type II diabetes patients.