The purposes of this article are to report the imaging finding of inferior displacement of the lower belly of the lateral pterygoid muscle (LPM) in patients with lesions arising from the temporomandibular joint (TMJ) and to propose that this imaging finding is suggestive of the presence of a TMJ lesion.We reviewed computed tomographic and magnetic resonance images of 9 patients with lesions involving the TMJ. Images were evaluated for identification of an inferiorly displaced lower belly of the LPM. Pathology reports were reviewed to determine the histopathologic diagnosis of the TMJ lesion.Inferior displacement of the lower belly of the LPM was observed in all cases on magnetic resonance images, computed tomographic images, or both. In 2 cases, the diagnosis was calcium pyrophosphate deposition disease. The remaining cases were ganglion cyst, joint effusion due to dermatomyositis, septic arthritis, chondromyxoid fibroma, synovial chondromatosis, pigmented villonodular synovitis, and giant cell tumor of bone.Inferior displacement of the lower belly of the LPM is a useful radiologic sign to suggest that a lesion arises from the TMJ.