This study tested if habit strength for taking oral hypoglycemic medication moderated the association between poor mental health symptoms (i.e. depressive symptoms, diabetes distress) and unintentional non-adherence in 790 adults with type 2 diabetes. Data were cross-sectional. A habit strength x depressive symptom interaction was observed. Depressive symptoms were negatively associated with adherence if habit was weak or of average strength; no association was observed if habit was strong. A similar interaction emerged when diabetes distress was examined. Results suggest that habit strength might operate as a buffer in the association between poor mental health symptoms and medication adherence.