Objective: To study the incidence of postoperative cardiac arrhythmias in children undergoing cardiac surgery and to evaluate the risk factors and outcome of these patients. Materials and Methods: This retrospective observational study was conducted in the cardiac pediatric intensive care unit and included children <18 years of age. Children were monitored in the early postoperative period (72 h) for any sustained rhythm abnormality and were classified using standard definition. Details of treatment and their response were assessed. Risk factors for arrhythmias were evaluated using multivariate logistic regression analysis. Results: Five hundred and thirty-six children were included and the prevalence of arrhythmia was 14.4% ( n = 77). The most common arrhythmia was complete heart block (CHB) ( n = 28; 5.2%), followed by junctional ectopic tachycardia (JET) ( n = 25; 4.7%), junctional escape rhythm ( n = 13; 2.4%), supraventricular tachycardia (SVT) ( n = 8; 1.5%), and ventricular tachycardia (VT) ( n = 3; 0.6%). Cardiac pacing was required in all CHB; 8 (28.6%) required a permanent pacemaker. Six (24%) patients with JET responded to conventional measures; 19 (76.0%) patients required amiodarone and 5 (20%) required cooling to 34°C or cardiac pacing. Temporary cardiac pacing was required in 9 (69.2%) cases of junctional escape rhythm. Seven (87.5%) events of SVT responded to adenosine and 1 (12.5%) required cardioversion. Two (66.7%) of VT responded to cardioversion while 1 (33.3%) was refractory. Five (6.5%) patients with arrhythmia died. In the multivariate logistic regression analysis, age <1 year, risk adjustment for congenital heart surgery category ≥3, and cross-clamp time >67 min were independent risk factors. Conclusion: Early postoperative period following cardiac surgery is extremely vulnerable to cardiac arrhythmias. Although majority are self-limiting, some can be life-threatening.