Background. Intrathyroidal ectopic thymus (IET), a benign lesion due to aberrant thymic migration during embryogenesis, is often discovered incidentally. We aimed to present the ultrasound (US) features, diagnostic methods, and follow-up of IET in children and adolescents. Methods. We searched our database of patients with a nodular thyroid lesion detected by US, between January 2007 and December 2019. In 30/255 (11.7%), IET was diagnosed. Results. The study included 30 patients (20 males/10 females), mean age 5 years (0.1-12.2, median 5.6) with 34 lesions diagnosed by US as `incidentalomas.` None of the patients had palpable nodules. On US, IET appeared as a hypoechoic lesion, with multiple punctuate internal echoes. 29/34 of lesions had well-defined margins. The most common location of IET was in the middle part (27/34) of the left lobe (19/34). The mean longest diameter at diagnosis was 6.4 mm (2.5–21, median 4.5). Sonographic follow-up was available in 25 patients with 27 lesions. The mean time of observation was 2.7 years (0.3-7.5, median 2.1). While 13/27 cases showed decreased size or regression during follow-up, the other 13 increased in size, and there was no change in size in one. Pubertal progression was associated with both increment and decrease in size of IET. Fine needle aspiration (FNA) was performed in 5 patients and surgery in one. Conclusions. IET should be considered in the differential diagnosis of pediatric thyroid nodules as a cause of FNA and/or surgery. Regular US monitoring can be used safely in the follow-up of this lesion. We present one of the largest series in the literature with long-term follow-up and description of patients` pubertal status. IET prevalence was 11.7% among children and adolescents with a nodular thyroid lesion, higher than that stated in the literature.