Objective: This study aimed to evaluate the efficacy of Elastographic Strain Index (SI) in the diagnosis and grading of pediatric Hashimoto’s thyroiditis (HT) patients. Methods: This cross-sectional, controlled trial was carried out in the pediatric radiology department of our tertiary care center. The study group consisted of 63 children with HT and control group was composed of 47 healthy children. Conventional ultrasonography (US) and SI elastography measurements were performed through the Aplio™ 500 ultrasonic device with a linear 4.8–11.0 MHz transducer and elastography software. Furthermore, the correlation between strain ratio values and other variables was investigated. Results: The mean age of the HT patients was 139.63 ± 27.61 months, and no significant difference was found between the control group (143.09 ± 27.32 months) and the patients regarding mean age and gender. The mean SI of normal healthy individuals and patients with HT was 0.26 ± 0.77 and 1.75 ± 1.46, respectively (p < 0.001). For the diagnosis for HT with elastography, the sensitivity was 92.1%, and specificity was 66% when the optimal mean SI cut-off value was taken as 0.31, (p < 0.001). Conclusions: The use of elastographic SI in patients with HT is a highly promising method for objective and countable results compared with the conventional US. The sensitivity of the increase in SI value was determined to be high in the diagnosis of HT. It was also considered that the elastography SI value had higher sensitivity and specificity than the conventional US in HT patients with moderate to advanced tissue hardening.