Objective There is limited information about the extent of metabolic syndrome (MetS) among subjects with cardiac diseases in Ethiopia. Therefore, this study aimed to determine the prevalence and associated factors of MetS among adults with cardiac diseases. Methods A cross-sectional study was conducted at Hawassa University Comprehensive Specialized Hospital in southern-Ethiopia between May 2021 and August 2021. Data pertaining to the study were collected from 319 participants (153 cardiac and 166 non-cardiac groups) using the WHO stepwise approach. The International Diabetes Federation (IDF) and the revised National Cholesterol Education Program: Adult Treatment Panel III (ATP) were used to define MetS. Results The prevalence of MetS was 44.2% and 50.2% in IDF and ATP III, respectively. Among the cardiac group, 54.9% had MetS in IDF criteria, compared to 34.3% in the non-cardiac group ( p < .001); whereas, it was 60.1% in the cardiac group and 41% in non-cardiac group ( p < .001) by ATP III. Age >60 years, waist–height ratio ≥5, high waist–hip ratio, body mass index 25–29.9 and ≥30 kg/m2 were significantly associated with higher odds of MetS in both IDF and ATP III. Also, smoking history and monthly income were associated with MetS by ATP III, while being female was associated with higher odds of MetS by IDF. Conclusion More than half of the subjects with cardiac diseases developed MetS as compared to non-cardiac controls. Periodic screening and focusing on the identified modifiable high-risk factors such as body weight, smoking, and central adiposity is important to effectively mitigate the risk of further comorbidity and complications in this high-risk population.