Background: Diabetes mellitus (DM) and cardiovascular disease (CVD) are present in a large number of patients with novel Coronavirus disease 2019 (COVID-19). We aimed to determine the risk and predictors of in-hospital mortality from COVID-19 in patients with DM and CVD.Methods: This retrospective cohort study included hospitalized patients aged ≥ 18 years with confirmed COVID-19 in Alborz province, Iran, from 20 February 2020 to 25 March 2020. Data on demographic, clinical and outcome (in-hospital mortality) data were obtained from electronic medical records. Self-reported comorbidities were classified into the following groups: “DM” (having DM with or without other comorbidities), “only DM” (having DM without other comorbidities), “CVD” (having CVD with or without other comorbidities), “only CVD” (having CVD without other comorbidities), and “having any comorbidity”. Multivariate logistic regression models were fitted to quantify the risk and predictors of in-hospital mortality from COVID-19 in patients with these comorbidities.Results: Among 2957 patients with COVID-19, 2656 were discharged as cured, and 301 died. In multivariate model, DM (OR: 1.62 (95%CI: 1.14-2.30)) and only DM (1.69 (1.05-2.74)) increased the risk of death from COVID-19; but, both CVD and only CVD showed non-significant associations (p>0.05). Moreover, “having any comorbidities” increased the risk of in-hospital mortality from COVID-19 (OR: 2.66 (95%CI: 2.09 -3.40)). Significant predictors of mortality from COVID-19 in patients with DM were lymphocyte count, creatinine and C-reactive protein (CRP) level (all P- values < 0.05).Conclusions: Our findings suggest that diabetic patients have an increased risk of in-hospital mortality following COVID-19; also, lymphocyte count, creatinine and CRP concentrations could be considered as significant predictors for the death of COVID-19 in these patients.