Background Patients with opioid use disorder are at a higher cardiovascular risk due to the effect of opioids on the cardiovascular system. Cardiac conduction abnormalities, electrical activity impairment, cardiac arrhythmias, and ventricular hypertrophy are reported in the opioid population. Objective This study aimed to assess the prevalence and factors associated with ECG abnormalities among adults with opioid use disorder attending the Itega addiction center for methadone-assisted therapy (MAT). Methodology A cross-sectional analytical study was conducted among adult outpatients attending the Itega addiction center in Dodoma. A calculated sample size of 321was attained through a convenience sampling approach. A standard 12-lead ECG was recorded for each participant and interpreted by two independent cardiologists. Univariate and multivariable logistic regression was computed to determine the factors associated with ECG abnormalities. Under adjusted analysis, a p-value of less than 0.05 was considered significant for factors associated with ECG abnormalities after controlling for all the variables with a minimum p-value of 0.2 at univariate analysis. Results The majority of 308 (95.95%) of the participants were males, 197 (61.37%) had attained primary education level, and the mean age of the participants was 35.44 ± 6.54 years. The overall prevalence for any ECG abnormalities in this study was 26.47%, with Sinus bradycardia 59(18.4%) being the most observed ECG abnormality, followed by QTc prolongation 27(8.41%). A month's increase in the duration on MAT and being a female were significantly associated with lower odds of ECG abnormalities (AOR =0.85, 95% CI =0.74-0.96 p =0.014) and (AOR = 0.05, 95% CI = 0.01-0.59, p = 0.017) respectively.  Conclusion The high prevalence of ECG Abnormalities implies high cardiovascular risk among a population with opioid use disorder. Given that majority of the ECG abnormalities are treatable, integrating cardiovascular care in the opioid addiction clinic would be beneficial for this population.