Acute injuries of the knee medial collateral ligament complex concomitant with anterior cruciate ligament injuries are common. The exact site of the injury may be difficult to diagnose preoperatively on magnetic resonance imaging. This study describes an arthroscopic sign that helps determine the site of the knee medial collateral ligament complex injury. The "medial compartment drive-through sign," visualized during arthroscopy, is described as an excessive opening of the medial compartment. If this excessive opening is above the meniscus, it corresponds to a femoral-sided injury; conversely, if the excessive opening is below the meniscus, then it is a tibial-sided injury. This allows a precise surgical incision to be made, thereby avoiding extensive approaches and possible wound-related complications.