Simple SummaryDiffuse sclerosing variant papillary thyroid carcinoma is a rare variant of papillary thyroid carcinoma that is most frequently observed in young patients with different clinical, pathological, and molecular profiles to classical PTC. Our findings revealed significant age-related differences in DSVPTC. DSVPTC was more aggressive in paediatric patients with a larger-sized tumour, more common multiplicity, and lateral neck metastasis. Despite being frequent and more aggressive in younger patients, DSVPTC also occurs in older patients with aggressive behaviour. Through targeted next-generation sequencing, we identified theBRAF,KRAS, andTERTmutations as the most important genes in DSVPTC with age-specific differences.AbstractDiffuse sclerosing variant papillary thyroid carcinoma (DSVPTC) is commonly observed in young patients, with a median age at diagnosis in the third decade of life. Further, the risk of recurrence is higher for DSVPTC than for classical PTC. Therefore, this study aimed to describe the clinicopathological and genetic characteristics of patients of different ages with DSVPTC. We retrospectively reviewed 397 patients who underwent thyroidectomy for DSVPTC at Gangnam Severance Hospital, Yonsei University, from January 2005 to December 2017. The mean age at diagnosis was 36.7 ± 11.6 years, with most patients (163, 41.1%) aged 31–40 years. DSVPTC was predominant in women (276, 69.5%). We observed recurrence in 46 (11.6%) patients, with regional nodal recurrence being the most common type of recurrence (32 patients, 69.6%). The mean tumour size was larger in younger patients than in older patients. DSVPTC was more aggressive in paediatric patients with a larger-sized tumour, more common multiplicity, and lateral neck metastasis. Through random sampling, we selected 41 patients by age group and examined the mutations in 119 genes using next-generation sequencing.BRAF,KRAS, andTERTdisplayed relatively higher mutation rates than other genes. DSVPTC displays different clinical, pathological, and molecular profiles than classical PTC. TheBRAF,KRAS, andTERTmutations are the most important, with age-specific differences.