In this retrospective study, we investigated the impact of diabetes mellitus (DM) on patients with head and neck cancer (HNC) undergoing concurrent chemoradiotherapy (CCRT). We analyzed the demographic and clinical characteristics, treatment tolerance, and toxicities of patients with HNC undergoing primary or adjuvant CCRT with or without DM between 2007 and 2016. Of the 556 patients undergoing CCRT, 84 (15.1%) had DM. Compared with patients without DM, patients with DM were significantly older (56.2 ± 11.2 vs. 51.9 ± 9.5 years, P < 0.001), received lower doses of cisplatin (adjuvant CCRT: 175.30 ± 84.03 vs. 214.88 ± 68.25, P = 0.014; primary CCRT: 142.84 ± 79.49 vs. 187.83 ± 76.19, P < 0.001), and experienced higher rates of infection (adjuvant CCRT: 52% vs. 30.5%, P = 0.042; primary CCRT: 45.8% vs. 22.9%, P < 0.001). Among patients undergoing primary CCRT, compared with those without DM, the patients with DM experienced significantly higher rates of hematologic toxicity (65.7% vs. 39.3%, P = 0.004) and treatment-related deaths (10.2% vs. 3.5%, P = 0.051); and a greater weight loss (-6.17 ± 9.27% vs. -4.49 ± 6.84, P = 0.078). Patients with HNC and DM undergoing CCRT, compared with patients without DM, experienced higher rates of infection and hematotoxicity, loss of body weight, and higher treatment-related mortality.