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Abstact Background and aims Initial proof suggests that a non-specific intraventricular conduction delay (NIVCD) is a risk factor for mortality. We explored the prognosis of intraventricular conduction delays (IVCD) – right bundle branch block (RBBB), left bundle branch block (LBBB), and the lesser-known NIVCD – in patients with acute coronary syndrome (ACS). Methods This is a retrospective registry analysis of 9,749 consecutive ACS patients undergoing coronary angiography and with an ECG recording available for analysis (2007–2018). The primary outcome was cardiac mortality. Mortality and cause of death data (in ICD-10 format) were received from the Finnish national register with no losses to follow-up (until December 31, 2020). The risk associated with IVCDs was analyzed by calculating subdistribution hazard estimates (SDH; deaths due to other causes being considered competing events). Results The mean age of the population was 68.3 years (SD 11.8). The median follow-up time was 6.1 years (IQR 3.3–9.4), during which 3,156 patients died. Cardiac mortality was overrepresented among IVCD patients: 76.9% for NIVCD (n = 113/147), 67.6% for LBBB (n = 96/142), 55.7% for RBBB (n = 146/262), and 50.1% for patients with no IVCD (n = 1,275/2,545). In an analysis adjusted for age and cardiac comorbidities, the risk of cardiac mortality was significantly higher in all IVCD groups than among patients with no IVCD: SDH 1.37 (1.15–1.64, p < 0.0001) for RBBB, SDH 1.63 (1.31–2.03 p < 0.0001) for LBBB, and SDH 2.68 (2.19–3.27) for NIVCD. After adjusting the analysis with left ventricular ejection fraction, RBBB and NIVCD remained significant risk factors for cardiac mortality. Conclusion RBBB, LBBB, and NIVCD were associated with higher cardiac mortality in ACS patients.