Cancer and diabetes mellitus (DM) are diagnosed within the same individual more frequently and share common risk factors. Although diabetes among cancer patients may result in more aggressive clinical courses of cancer, there is limited evidence about its burden and associated factors. Hence, this study aimed to assess the burden of diabetes and prediabetes among cancer patients and its associated factors. Institution-based cross-sectional study was conducted at the University of Gondar comprehensive specialized hospital from 10 January to 10 March 2021. A systematic random sampling technique was used to select 423 cancer patients. The data was collected using a structured interviewer-administered questionnaire. Prediabetes and diabetes diagnosis was made based on World Health Organization (WHO) criteria. Bi-variable and multivariable binary logistic regression models were fitted to identify factors associated with the outcome. Adjusted Odds Ratio (AOR) with a 95% confidence interval was estimated to show the direction and strength of associations. Variables with a p-value less than 0.05 in the multivariable model were considered significantly associated with the outcome. The final analysis was based on 384 patients with cancer. The proportion of prediabetes and diabetes was 56.8% (95% CI 51.7, 61.7) and 16.7% (95% CI 13.3, 20.8), respectively. Alcohol consumption was found to increase the odds of elevated blood sugar among cancer patients (AOR: 1.96; 95%CI: 1.11, 3.46). The burden of prediabetes and diabetes is alarmingly high among cancer patients. Besides, alcohol consumption was found to increase the odds of having elevated blood sugar among cancer patients. Hence, it is essential to recognize cancer patients are at high risk of having elevated blood sugar and design strategies to integrate diabetes and cancer care.