Aim: To describe a rare case in which a patient developed severe trismus associated with pain and paresthesia on the left side of the face, which, on comprehensive assessment and findings from fine needle aspiration cytology (FNAC), computed tomography (CT), and positron emission tomography-computed tomography (PET-CT), revealed metastasis involving left lateral pterygoid muscle from lung cancer. Background: Metastatic tumors involving the oral cavity are rarely encountered in clinical practice and act as a diagnostic challenge to the clinician. The presence of trismus as a clinical symptom can hamper the diagnosis, as it can lead to misjudgment, thereby hindering the early identification of the underlying malignancy and delaying timely treatment for the patient. Case description: A 45-year-old male, presented with the chief complaint of trismus and pain in the left side of the face for the past 5 months, associated with paresthesia. The patient also gave a history of weight loss with no other significant medical history. He had a habit of smoking cigarettes and alcohol consumption since 30 years. On comprehensive examination, a single left submandibular lymph node was palpable, and also tenderness in masseter, temporalis, and sternocleidomastoid muscle on the left side was present. Intraorally, there was tenderness in the buccal vestibule of mandibular left posterior tooth region with no other significant soft tissue and hard tissue findings. Blood investigation reports, orthopantomogram (OPG), and chest X-ray did not reveal any pathology. Based on the above findings, a provisional diagnosis of pterygomandibular space infection secondary to periodontal infection from the mandibular left third molar was considered. A full course of antibiotics administration did not cause any improvement; hence, a CT scan of the head and neck was advised, which unveiled a peripherally enhancing hypodense lesion with central hypodensity involving left lateral pterygoid muscle and further specialized investigations like FNAC and PET-CT revealed primary malignant lesion of the lung with metastasis to left lateral pterygoid muscle and left cervical lymph nodes. A teatment plan of chemotherapy in combination with radiotherapy was devised at a regional oncology center. However, due to the advanced stage of the condition and poor prognosis, the patient succumbed to the disease within 6 months of initiating treatment. Conclusion: This is one of the unique cases of metastasis to lateral pterygoid muscle from lung carcinoma with the clinical presentation of trismus, which mimicked odontogenic infection or a temporomandibular joint disorder. This case report also emphasizes the role of CT and PET-CT in diagnosing the primary malignant lesion and its metastasis to distant sites. Clinical significance: This is a rare case where metastasis to the oral and maxillofacial region manifested as trismus, a rare sign that was the first clinical indication of an undiscovered distant primary tumor. This case highlights the critical role of comprehensive clinical assessment and advanced imaging in identifying underlying malignancies in patients with seemingly innocuous symptoms like trismus and facial pain.