Partial atrioventricular septal defect (AVSD) is a congenital heart anomaly that typically manifests in childhood and rarely presents in elderly patients. However, this anomaly can lead to hemodynamically significant cardiopulmonary complications when left untreated. In this report, we present a case of a 68-year-old male patient who presented with symptoms of heart failure and atrial flutter and was found to have partial AVSD incidentally discovered by echocardiography. He was stabilized with diuretics and beta-blockers, and anticoagulation therapy was initiated, given his risk for thromboembolism as per his CHA2DS2VASc2 score. He was subsequently referred for surgical repair, which he declined, opting for medical therapy. The patient was regularly followed up as an outpatient with improvement of symptoms on medical therapy. This case highlights the severe complications of unrepaired partial AVSD in adulthood, highlighting the need for early diagnosis and intervention, since late identification in adulthood can result in significant morbidity. However, in cases where surgery is not possible or declined, patients can be adequately optimized medically to improve overall quality of life and decrease morbidity and mortality associated with this untreated anomaly.