A dermoid cyst arising in the left lobe of the thyroid gland in a 5-year-old boy was managed with a left hemi-thyroidectomy. While congenital lesions feature frequently in the differential diagnosis of childhood neck masses, dermoid cysts presenting as a unilateral neck mass are rare. When found within the thyroid, their pathogenesis may be explained by a disruption of normal thyroid development. Excision appears the treatment of choice.