Background: Acute coronary syndrome is increasingly becoming a common cause of cardiovascular mortality in developing countries. Even though, there is an introduction of limited percutaneous coronary intervention and thrombolytic therapies, in-hospital mortality due to acute coronary syndrome still remains high in Ethiopia. Objectives: To assess the clinical features and treatment outcome of patients with acute coronary syndrome admitted to the emergency department of Saint Paul’s Hospital Millennium Medical College (SPHMMC), Addis Ababa, Ethiopia. Methods: An institutional based retrospective cross sectional study was conducted at Saint Paul’s hospital emergency department. All patients who had a final diagnosis of acute coronary syndrome and fulfilled the inclusion criteria were included in the study. The collected data was entered & analyzed using statistical package for social science(SPSS) version 23. Descriptive statistics was used to describe the variables in the study and the result was presented with text, figures and tables. Results: Analysis was performed for 170 patients which fulfilled the inclusion criteria. There was a strong male predominance. The mean age of patients was 54.65 ± 14.45 years. The main risk factor was hypertension, found in 53.5% of cases and the most common presenting symptom was chest pain found in 90.6% of patients. Diagnosis of ST- elevation myocardial infarction in 71.8% of patients and non-ST-elevation acute coronary syndrome in 28.2% was made by the combination electrocardiographic features and troponin assay. Heart failure was the commonest complication seen in 18.8% of the patients. In-hospital mortality was 25.2%. Conclusions: Acute coronary syndrome affected patients in this study at a relatively younger age than the western population and their presentation to the emergency department was very late leading to high mortality rates.