Background: Childhood cancer survivors (CCS) are at risk for radiotherapy (RT) late effects, including second malignancies. Optimal screening for thyroid cancer (TC) in CCS post-RT remains controversial. We assessed the clinical benefit of thyroid ultrasound (US) surveillance in CCS exposed to RT. Methods: 316 CCS (175 males) were prospectively surveilled with thyroid US between 2002 and 2021 at the Children’s Hospital of Philadelphia. Patients were screened upon referral to the Survivorship Program. Thyroid US, clinicopathologic features, and endocrine-related outcomes were ascertained. Outcomes were compared using primary CCS diagnosis age cohorts of ≤ 3, > 3 to ≤ 10, and > 10 years. Risk factors for thyroid nodule(s) and TC were evaluated using Kruskal Wallis and ANOVA [OR (95% CI)]. Results: The most common CCS diagnoses were leukemia (32%), CNS tumor (26%), and neuroblastoma (18%). Patients received TBI (43%) and/or RT to craniospinal (43%), chest (13%), and neck regions (7%). About 48% (n = 152) of patients presented thyroid nodule(s) (Table). Forty-six patients underwent surgery, and 28 had TC, including 19 with ATA low-risk, 2 with ATA intermediate-risk, and 7 with ATA high-risk disease. Of the 9 patients with intermediate- or high-risk disease, 5 were ≤ 3 years, 3 were > 3 to ≤ 10 years, and 1 was > 10 years at the time of RT exposure. Eight patients with TC demonstrated pathogenic variant(s). RT exposure at ≤ 3 years old conferred 2-fold increased risk for nodule(s) compared to RT at > 10 years [OR = 2.14 (1.44-2.84) p = 0.03]. Female sex [OR = 1.73 (1.25-2.21) p = 0.02] and greater interval between RT and first US [OR = 1.10 (1.04-1.15) p = 0.001] were additional independent risk factors. Conclusions: Younger age at RT exposure is associated with increased risk of and shorter latency for developing TC. Thyroid US surveillance appears most beneficial in CCS exposed to RT ≤ 3 years old in an effort to diagnose TC at an earlier stage prior to metastasis.