There is growing evidence that patients with type 2 diabetes mellitus (DM) have an increased risk of developing Parkinson's disease (PD) and share similar dysregulated pathways. We aimed to determine whether the risk of PD increases as diabetes progresses among patients with type 2 DM. Using a nationally representative database from the Korean National Health Insurance System, 2,362,072 individuals (≥40 years of age) with type 2 DM who underwent regular health checkups during 2009-2012 were followed up until the end of 2018. The diabetes severity score parameters included the number of oral hypoglycemic agents, diabetes duration, insulin use, or presence of chronic kidney disease, diabetic retinopathy, or cardiovascular disease. Each of these characteristics was scored as one unit of diabetes severity and their sum was defined as a diabetes severity score from 0-6. We identified 17,046 incident PD cases during the follow-up. Each component of the diabetes severity score showed a similar intensity for the risk of PD. Compared with subjects with no parameters, HR values (95% confidence intervals) of PD were 1.09 (1.04-1.15) in subjects with one diabetes severity score parameter, 1.28 (1.22-1.35) in subjects with two parameters, 1.55 (1.46-1.65) in subjects with three parameters, 1.96 (1.82-2.11) in subjects with four parameters, 2.08 (1.83-2.36) in subjects with five parameters, and 2.78 (2.05-3.79) in subjects with six parameters. Diabetes severity was associated with an increased risk of developing PD. Severe diabetes may be a risk factor for the development of PD.