The sinus venosus (SV) plays a significant role in the embryological heart as the initial structure where the cardinal, umbilical, and vitelline veins drain before remodeling into the caval veins. As the human heart develops, the SV incorporates into the posterior wall of the right atrium. Sinus venosus atrial septal defects (SVASDs) result from a defect in the wall present among the right pulmonary veins, the superior vena cava (SVC), and the right atrium. Persistent left superior vena cava (PLSVC) occurs when the Marshall ligament does not regress, and in most cases, the PLSVC enters the coronary sinus before draining into the right atrium. Pulmonary hypertension from chronic left to right shunting makes recognizing this condition clinically significant. In this case report, both cardiac CT and transesophageal echocardiogram were used to further evaluate an SVASD with partial anomalous pulmonary venous return (PAPVR) of the right superior pulmonary vein, in addition to a PLSVC. The incidence of the co-occurrence of SVASD and PLSVC, as well as the association between the two, were discussed in this case report. Future research should focus on the potential genetic causes of this co-occurrence. It should also focus on patient treatment and outcomes at different stages of presentation to optimize patient management and improve mortality.