A 12-year-old boy presented with tachycardia and cardiomegaly. An electrocardiogram showed atrial flutter with a ventricular rate of 170 beats/min. The echocardiogram showed left ventricular dilation, with decreased contractility. Electromyography confirmed the diagnosis of myotonic dystrophy. The atrial flutter was converted to sinus rhythm by electroversion, leading to restoration of normal cardiac function. Our case illustrates that atrial flutter can be an early sign of the cardiac complications of myotonic dystrophy in childhood.