Introduction and objective: Many studies have investigated the interplay between Parkinson’s disease and diabetes mellitus, suggesting that glucose metabolism impairment may worsen the clinical course of Parkinson’s disease. This study aimed to explore the association between diabetes mellitus and the course of Parkinson’s disease. Materials and methods: A retrospective study was performed by analysing the clinical data of patients diagnosed with Parkinson’s disease who were hospitalised in University Clinical Centre of the Medical University of Silesia from 2019 to 2021. The study group comprised 241 patients selected according to the study’s inclusion and exclusion criteria. Their clinical conditions were assessed using body mass index, the Movement Disorder Society Unified Parkinson’s Disease Rating Scale (MDS-UPDRS), the Hoehn–Yahr scale, Mini-Mental State Examination (MMSE), Clock Drawing Test (CDT) and Beck Depression Inventory (BDI). Data were collected on current anti-parkinsonian treatment, fasting glycaemia, lipid panel, and thyroid stimulating hormone, homocysteine and vitamin D₃ levels. Results: The study group included 31 patients diagnosed with diabetes mellitus and 240 patients without glucose metabolism impairment. Both groups were matched by considering age, disease duration, and gender distribution. Diabetic patients displayed a higher MDS-UPDRS part III OFF rating (42 [31–55] vs. 48 [39–59]; p = 0.0043), higher MDS-UPDRS part III ON rating (17 [11–26] vs. 26.5 [19–32]; p = 0.0009) and higher BDI score (7 [4–11] vs. 11 [9–16]; p = 0.0013). As expected, patients with diabetes mellitus had a higher fasting glycaemia, total cholesterol, lower high-density lipoprotein cholesterol and higher body mass index. Conclusions: Our study confirms the relationship between diabetes mellitus and a worse clinical course for Parkinson’s disease.