Cervical ectopic thymus (CET) is one of the rare causes of cervical masses in childhood. These lesions manifest with the same echo and signal intensity as the natural thymus on ultrasonography (US) and magnetic resonance imaging (MRI), respectively. Therefore, both US and MRI are appropriate imaging methods for the diagnosis of CET. We evaluated the US and MRI features of the CET patient who presented to our hospital due to bilateral cervical swelling. In particular, based on the typical US appearance of thymic tissue, we believe that we can safely prevent the need for biopsy to detect that a discovered lesion that has ectopic thymus tissue. In this article, we aimed to emphasize the importance of imaging methods in the diagnosis of CET in a pediatric patient with a complaint of neck mass.