Background: Diabetes mellitus is known to exacerbate bacterial infection, but its effect on the severity of viral infection has not been well studied. The severity of thrombocytopenia is an indicator of the severity of dengue virus infection. We investigated whether diabetes is associated with thrombocytopenia in dengue-infected patients. Methods: We studied clinical characteristics of 644 patients with dengue infection at a university hospital during the epidemic on 1 June 2002 to 31 December 2002 in Taiwan. Platelet counts and biochemical data were compared between patients with and without diabetes. Potential risk factors associated with thrombocytopenia were explored using regression analyses. Results: Dengue-infected patients with diabetes had lower platelet counts than patients without diabetes during the first three days (54.54 ± 51.69 vs. 86.58 ± 63.4 (p ≤ 0.001), 43.98 ± 44.09 vs. 64.52 ± 45.06 (p = 0.002), 43.86 ± 35.75 vs. 62.72 ± 51.2 (p = 0.012)). Diabetes mellitus, death, dengue shock syndrome (DSS) and dengue hemorrhagic fever (DHF) and increased glutamic-pyruvate transaminase (GPT) levels were significantly associated with lower platelet counts during the first day of hospitalization for dengue fever with regression β of −13.981 (95% confidence interval (CI) −27.587, −0.374), −26.847 (95% CI −37.562, −16.132), and 0.054 (95% CI 0.015, 0.094) respectively. Older age, hypoalbuminemia, and hypertriglyceridemia were independently correlated with thrombocytopenia in dengue patients with or without diabetes with regression β of −2.947 (p = 0.004), 2.801 (p = 0.005), and −3.568 (p ≤ 0.001), respectively. Diabetic patients with dengue had a higher rate of dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS) than non-diabetic patients. They also had lower blood albumin, were older, and higher triglyceride levels. Older age, hypoalbuminemia, and hypertriglyceridemia were independently correlated with thrombocytopenia in dengue patients. Conclusions: Dengue patients with diabetes tended to have more severe thrombocytopenia and were more likely to have DHF/DSS. Older age, hypoalbuminemia, and hypertriglyceridemia were independently associated with more severe thrombocytopenia in dengue patients.