This study reports the prevalence and prognostic impact of right bundle branch block (RBBB) and left bundle branch block (LBBB) in the admission electrocardiogram (ECG) of comatose survivors of out-of-hospital cardiac arrest (OHCA). The present study is part of the predefined electrocardiographic substudy of the prospective randomized target temperature management trial, which found no benefit of targeting 33°C over 36°C in terms of outcome. Six-hundred eighty-two patients were included in the substudy. An admission ECG, which defined the present study population, was available in 602 patients (88%). These ECGs were stratified by the presence of LBBB, RBBB, or no-BBB (reference) on admission. End points were mortality and neurologic outcome 6 months after OHCA. RBBB was present in 79 patients (13%) and LBBB in 65 patients (11%), and the majority of BBBs (92%) had resolved 4 hours after admission. RBBB was associated with significantly higher 6 months mortality (RBBB: hazard ratio [HR]_unadjusted 1.78, 95% confidence interval [CI] 1.30 to 2.43; LBBB: HR_unadjusted 1.26, 95% CI 0.87 to 1.81), but this did not reach a level of significance in the adjusted model (HR_adjusted 1.33, 95% CI 0.94 to 1.87). Similar findings were seen for neurologic outcome in the unadjusted and adjusted analyses. RBBB was further independently associated with higher odds of unfavorable neurologic outcome (RBBB: adjusted odds ratio 1.97, 95% CI 1.05 to 3.71). In conclusion, BBBs after OHCA were transient in most patients, and RBBB was directly associated with higher mortality and independently associated with higher odds of unfavorable neurologic outcome. RBBB is seemingly an early indicator of an unfavorable prognosis after OHCA.