Pacemaker induced tachycardia (PIT) is a rare iatrogenic rhythm disorder which tipically occurs in patients with dual-chamber pacemakers and has different causes, including oversensing of atrial tachyarrhythmia waves. Cor triatriatum sinistrum is a congenital abnormality infrequent in adults, associated with a high risk of atrial tachyarrhythmia. We present the case of a 80-year-old woman incidentally diagnosed with cor triatriatum sinistrum echocardiographycally, implanted with a DDD pacemaker for sinus node disease, who developed atrial tachyarrhythmia (paroxysmal atrial fibrillation and left atrial tachycardia), which triggered a PIT, successfully aborted by automatic mode switch. This is the first case in literature that associates cor triatriatum sinistrum and PIT.