Interatrial shunting is a proposed technique to reduce elevated left heart and pulmonary pressures in heart failure patients. Clinical trials show promising results in relief of symptoms and improvements in quality of life, but little is still known about the working principles of interatrial shunting and important questions remain regarding the optimal diameter. The current research investigates the patient-specific optimal diameter of an interatrial shunt through computational fluid dynamics simulations. An idealized two-dimensional model of the left and right atria, four pulmonary veins, the two vena cavae and the two atrioventricular valves is used to study the intra- and interatrial flow fields in absence and presence of an interatrial shunt through steady-state and transient simulations. For the transient simulations, the inlets and outlets are coupled through a Windkessel including four reservoirs, representing the two ventricles and the pulmonary and systemic circulations. This coupling represents the closed-loop behavior of the circulatory system and ensures realistic inlet and outlet pressures throughout the cardiac cycle. Furthermore, wall movement is applied in the transient simulation to model the atrial deformation during the systolic and diastolic phases of the heart cycle. The introduction of an interatrial shunt significantly influences the atrial flow field and a shunt flow from the left atrium to the right atrium is observed in all the simulations throughout the cardiac cycle, increasing the ratio of pulmonary to systemic blood flow (Qp:Qs). This reduces pulmonary, left atrial and left ventricular pressures while slightly increasing systemic, right atrial and right ventricular pressures. The patient-specific optimal shunt diameter is studied by applying shunts with diameters ranging between 0 and 20 mm to three patients of varying left ventricular stiffness. From this, it is found that the pressure reductions of the smallest (< 6 mm) as well as the largest shunts (> 12 mm) are little sensitive to shunt diameter, whereas medium-sized shunts are the most sensitive to the diameter. It is concluded that the optimal shunt diameter is patient-specific and is defined as the smallest diameter that manages to reduce a patient’s peak pulmonary pressure to below 15 mmHg, as long as its Qp:Qs ratio does not exceed 1.5. Otherwise, the optimal shunt is the one with the largest diameter that does not exceed Qp:Qs = 1.5.