Cystic ventral cervical masses in cats are uncommon. Differential diagnosis includes branchial, thyroglossal, thyroid and parathyroid cyst, thyroid and parathyroid cyst-adenoma, parathyroid and thyroid adenocarcinoma, salivary mucocele, and abscess. Location of the mass, cytology, selected hormonal assays of cystic fluid, utrasonogram and thyroid scintigraphy can be helpful in narrowing down a diagnosis. The following case report on a fifteen-year-old cat with cystic thyroid adenocarcinoma, discusses the diagnostic workup, differential diagnosis, and treatment of cystic cervical masses in cats with emphasis on thyroid adenocarcinoma.