To investigate the association between diabetes and HCV infection in persons living with HIV and to determine the impact of diabetes on the occurrence of serious liver events (SLEs) and liver-related deaths (LRDs) among HIV/HCV-co-infected patients. Patients were included if they had at least one follow-up visit. In a cross-sectional analysis among all HIV patients, we have investigated the association between diabetes and HCV infection. A further longitudinal analysis was performed in the population of HIV/HCV-co-infected free from SLE with FIB-4 index <3.25 at baseline, using the following endpoints: (A) first event between SLE and LRD; (B) liver fibrosis progression defined as the first of two consecutive FIB-4 > 3.25; (C) first event between SLE, LRD, and liver fibrosis progression. Data from 15,571 HIV patients were analyzed: 2944 (18.9%) were HCV-Ab positive, and 739 (4.7%) presented a diagnosis of diabetes at their last follow-up. Among HIV/HCV-co-infected population, 107 patients had a diagnosis of diabetes. Viremic HCV-co-infected patients had 3-fold risk of diabetes onset than HCV-uninfected patients. On HIV/HCV-co-infected population, 85 SLEs/LRDs occurred over 20,410 person-years of follow-up (PYFU), for an incidence rate of 4.2/1000 PYFU (95%CI 3.4–5.2). Diabetic patients had 3-fold risk of pooled SLE and LRD than patients without diabetes. Furthermore, viremic HCV infection was independently associated with a higher risk of SLE/LRD (aIRR 3.35 [95%CI 1.14–9.83]). In HIV-infected patients, viremic HCV co-infection is a strong predictor of diabetes. Among HIV/HCV-co-infected population, diabetic patients showed an increased risk of SLE/LRD compared with those without diabetes. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.