The prevalence of ectopic thyroid tissue as consequence of an aberrant migration of thyroid during embryogenesis ranges up to 10% in autopsy studies. The differential diagnosis between the relatively rare occurrence of a primary carcinoma arising in ectopic thyroid tissue and the more frequent presence of cervical lymph node metastasis from papillary thyroid carcinoma (PTC) might represent a difficult challenge in the clinical practice. The clinical relevance of these lesions lies in their risk of hidden primary thyroid cancer. Our intention is to provide in this review the current limited data available and to report an unusual localization of primary PTC arising from an extra-thyroid area, responsible for a solitary cervical mass as initial manifestation. The tumor developed in an ectopic thyroid tissue embedded within the clavicular head of the sternocleidomastoid muscle and was completely separated from the thyroid. Surgical excision of ectopic thyroid tissue with clavicular head of sternocleidomastoid muscle along with total thyroidectomy and central and selective lateral neck dissection were carried out. Histopathology was diagnostic for ectopic PTC and no primary lesions in the thyroid gland neither metastatic lymph nodes were found. Tumor cells were positive for thyroid transcription factor-1and thyroglobulin, and negative for CD56. A postoperative adjuvant radioiodine ablation was given after recombinant human thyroid-stimulating hormone (TSH) stimulation and the post-treatment whole body scan was negative. After the evaluation at six months showing negative neck ultrasound and undetectable thyroglobulin levels, while TSH suppressed and after recombinant human TSH stimulation, the patient was re-evaluated every six months. At two years, the patient remained completely free of disease and is currently on substitutive dose of l-thyroxine. Endocrinologists and neck surgeons must be aware of the rare possibility of primary PTC arising from ectopic thyroid tissue within the sternocleidomastoid muscle.