Background: Acute Coronary Syndrome (ACS) and pneumonia are significant contributors to global morbidity and mortality. This study aims to investigate the relationship between pneumonia incidence and 30-day mortality in ACS patients. Methods: This retrospective cohort study analyzed data from 439 ACS patients admitted to Dr. Wahidin Sudirohusodo Hospital, Makassar, from August 2021 to July 2022. Patients were categorized based on pneumonia diagnosis and followed up for 30 days post-ACS. The primary outcome was 30-day mortality. Results: Of 439 ACS patients, 94 (21.4%) had pneumonia. The 30-day mortality rate was significantly higher in ACS patients with pneumonia (39.1%) compared to those without (19.3%) (RR: 2.681, 95% CI: 1.41 5.1, p=0.002). Patients with pneumonia, diabetes mellitus, and chronic kidney disease had the highest risk of 30-day mortality (RR: 58.8, 95% CI: 6.9- 500.69, p<0.001). Conclusion: Pneumonia significantly increases the risk of 30-day mortality in ACS patients, especially when combined with other comorbidities. These findings emphasize the importance of pneumonia prevention and management in ACS patients to improve outcomes.