BACKGROUND : The incidence of a secondary solid malignancy after hematopoietic cell transplantation (HCT) is increasing in long-term survivors. OBJECTIVE : The aim of this study was to compare the clinicopathological characteristics of HCT recipients with secondary thyroid cancer (STC), with those of non-HCT thyroid cancer patients. METHODS : We retrospectively investigated 5184 patients who received HCT between 1983 and 2016. Of these, 18 patients developed STC and underwent thyroidectomy due to differentiated thyroid cancer. We compared the clinicopathological characteristics of post-HCT thyroid cancer patients (post-HCT group) with those of a primary differentiated thyroid carcinoma cohort (cohort group) from Seoul St. Mary's Hospital. RESULTS : The mean ages at HCT and thyroidectomy after HCT were 22.0 and 31.8 years, respectively, and the median time interval between HCT and thyroidectomy was 5 years (range 1-16). Compared with the cohort group, the post-HCT group was younger at cancer onset and frequently had a palpable mass at initial diagnosis. The post-HCT group had more aggressive features, including larger tumor size, frequent extrathyroidal extension, lymphatic invasion, perineural invasion, and frequent lateral neck node metastasis and distant metastasis, relative to the cohort group; however, most patients (83.2%) in the post-HCT group were stage I or II. Additionally, BRAF V600E mutation was less frequent in the post-HCT group. CONCLUSIONS : We found that STC after HCT showed younger presentation and more aggressive clinical presentation. Therefore, a policy of regular screening, including neck ultrasound examination, may promote early detection and treatment in HCT recipients.