Preauricular swelling followed by limited mouth opening can be a challenging diagnosis. All anatomical structures and related pathologies should be examined. A 41-year-old woman presented to the Emergency Room complaining of limited mouth opening, severe pain that was worse on chewing, and swelling over her right temporomandibular joint (TMJ) that had started 3 days previously. Physical examination revealed localized swelling and redness over her right TMJ. Her right masticatory muscles were very painful and tender on palpation. Mouth opening was slightly limited. A computed tomography (CT) scan demonstrated a collection adjacent to her right TMJ, and reactive arthritis was diagnosed. Aspiration and arthrocentesis were performed, leading to immediate improvement. When symptoms returned 6 months later, magnetic resonance imaging (MRI) demonstrated a cystic lesion adjacent to the TMJ. Under general anesthesia an excisional biopsy was performed, and the pathologic examination revealed fibrous connective tissue with pseudocyst-like formation compatible with ganglion cyst. This case report demonstrates that a CT scan might lead to an incorrect diagnosis, whereas MRI scanning led to the correct diagnosis, thereby emphasizing the importance of choosing the right imaging modality. When a pathologic process of the TMJ is suspected, MRI is the gold standard imaging modality to diagnose the exact TMJ pathology. Complete surgical excision of ganglion cyst is recommended to determine a definite diagnosis. The final diagnosis should eventually be made by combining standard histology and MRI findings. Schlagwörter: ganglion cyst, reactive arthritis, temporomandibular disorders (TMD), temporomandibular joint (TMJ)