Objective: This study investigated the prevalence of generalized anxiety disorder (GAD) in Taiwanese patients with type 2 diabetes mellitus (T2DM). Methods: This retrospective observational study was conducted with a random sample of patients from the entire population of National Health Insurance enrollees during 2000-2010 and used ICD-9-CM diagnostic codes to identify T2DM patients and GAD. The prevalence of GAD was compared between T2DM patients and the general population. Results: Between 2000 and 2010, the prevalence of GAD was significantly greater in the T2DM patients than the general population, while the increase of GAD was higher in the general population (from 0.25 to 0.63%) than among T2DM patients (from 0.81 to 1.03%). In T2DM patients, GAD was associated with female gender, a Charlson Comorbidity Index ≥ 1, diabetes mellitus duration > 9 years, and the following comorbidities: congestive heart failure, peripheral vascular disease, and depressive disorder. The prevalence of GAD among T2DM patients was negatively associated with rapid-acting insulin injection therapy and with the use of metformin and sulfonylureas. Conclusion: Since the prevalence of GAD was greater among T2DM patients than the general population, public health initiatives are needed to prevent and treat GAD in T2DM patients, specifically those with the above mentioned risk factors.