Background <p>The parathyroid glands are small but essential components of the endocrine system, regulating calcium and phosphate homeostasis through the secretion of parathyroid hormone. Their anatomical location is highly variable, ranging from the carotid bifurcation superiorly to the mediastinum inferiorly. This report presents a rare case of an ectopic parathyroid adenoma located entirely posterior to the left submandibular gland in a patient with primary hyperparathyroidism.</p> Methods <p>The patient underwent physical and laboratory evaluation followed by cervical ultrasound, planar scintigraphy, SPECT/CT, 4D CT, and ¹⁸F-fluorocholine PET/CT. A left parathyroidectomy was subsequently performed under general anesthesia. In addition, a literature review covering the past 40&#xa0;years was conducted to identify reports of parathyroid adenomas located behind the submandibular gland.</p> Results <p>Cervical ultrasound, planar scintigraphy, and SPECT/CT were reported as negative for the presence of pathological parathyroid tissue. A solid lesion was identified directly posterior to the left submandibular gland on 4D CT. On ¹⁸F-fluorocholine PET/CT, the lesion demonstrated focal radiotracer uptake. The definitive diagnosis of an ectopic parathyroid adenoma was established by histopathological examination after surgical excision. A literature search identified three manuscripts relevant to the study objective; however, one was only partially applicable, while the remaining two provided limited imaging data.</p> Conclusion <p>Ectopic parathyroid adenomas in the submandibular region are uncommon, and as demonstrated by the literature review, complete posterior positioning behind the submandibular gland is exceptionally rare. This anatomical configuration reflects incomplete embryological descent of the parathyroid glands and, despite its rare occurrence, should be consistently considered in the diagnostic assessment of patients with primary hyperparathyroidism.</p> Graphical Abstract <p></p>