Background Developing countries are suffering from the previously existing infectious diseases and alarmingly growing burden of noncommunicable diseases like diabetes mellitus. There is increased speculation that diabetes mellitus might attribute to high infectious diseases burden, such as tuberculosis. The global importance of diabetes mellitus as a tuberculosis-risk factor is still not a well-established fact. Thus, we conducted this study to determine the prevalence of diabetes mellitus and its associated factors among adult tuberculosis patients attending tuberculosis clinics. Methodology We conducted a cross-sectional survey, from March 10 to April 15, 2017, among 421 tuberculosis patients receiving tuberculosis treatment in health facilities of Dire Dawa City Administration Council, Eastern Ethiopia. Study participants were selected using systematic random technique, and data were collected using a structured questionnaire. Fasting blood sugar and anthropometric measurements were carried out for all participants. A logistic regression analysis was performed to identify factors associated with diabetes mellitus. Result The prevalence of diabetes mellitus in this study was 13.5%. Age 26–40 (AOR = 6, 95% CI: (1.28, 27.5)), age ≥41(AOR = 9, 95% CI: (1.9, 44.4)), and family history of diabetes (AOR = 3.14, 95% CI: (1.23, 8.02)) were found to have a significant association with diabetes mellitus. Conclusion This study found that the magnitude of diabetes mellitus among tuberculosis patients was higher than the national estimated prevalence of diabetes mellitus in Ethiopia. This study suggests the need for screening each tuberculosis patient for diabetes.