We report identification of a prominent Thebesian valve by cardiovascular computed tomography (CT) angiography impeding cannulation of the coronary sinus, with subsequent successful coronary venous lead placement with cannulation of the coronary sinus ostium via a transvenous femoral vein approach and subsequent cannulation of the ostium with the coronary venous lead with a left subclavian approach. A 57-year-old man with nonischemic dilated cardiomyopathy, New York Heart Association Class III heart failure, left bundle branch block, and an ejection fraction of 15%, underwent an attempted cardiac resynchronization therapy implantable cardiac defibrillator (ICD). As the coronary sinus ostium could not be cannulated, a dual chamber ICD was placed. The patient subsequently underwent cardiovascular CT angiography, which identified a prominent Thebesian valve at the coronary sinus ostium as the anatomic obstacle to cannulation. Reattempted transvenous cardiac resynchronization therapy was accomplished successfully with a double cannulation approach: cannulation of the coronary sinus ostium with a catheter via a transvenous femoral vein approach and subsequent cannulation with the coronary venous lead via a left subclavian approach. When a prominent Thebesian valve is identified as an obstacle to transvenous left ventricular lead placement, cannulation of the coronary sinus by an alternate venous approach may allow for a coronary venous route rather than necessitate an epicardial approach.