Ectopic thyroid tissue in the lateral neck is a rare finding, especially in the submandibular region. This case report presents a 38-year-old female patient with swelling in the lateral cervical neck. Due to a thyroid goitre, right hemithyroidectomy was performed in the past. However, a persistent high thyroglobulin level was detected after surgery. Regarding the suspected tumour in the submental region, a cervical magnetic resonance imaging (MRI) was performed, which revealed a suspicious looking mass. The patient underwent complete surgical excision and the histopathological report concluded that the tumour was ectopic thyroid tissue. Her thyroglobulin level decreased back to a normal level after excision of the submandibular mass. These results show that ectopic thyroid tissue must be considered a differential diagnosis for patients with unclear swelling in the submental region.