Objective: Type 1 diabetes mellitus (DM) is associated with autoimmune diseases such as thyroiditis. Therefore, we aimed to investigate the prevalence of autoimmune thyroiditis in patients with type 1 DM. Methods: A total of 102 patients who were diagnosed and followed up (average duration: 8.1 ± 4.0 years) in Ajou University Hospital were enrolled in this study. All the patients were evaluated for beta cell autoimmunity, including insulin autoantibody, glutamic acid decarboxylase antibodies (GADA), and islet cell antibody. Moreover, autoantibodies to thyroid peroxidase and thyroglobulin were assessed at the initial diagnosis and thereafter, were annually checked. Results: The mean patient age (49 men and 53 women) was 19.2 ± 4.8 years. The prevalence of at least one thyroid autoantibody was 30.4%. Patients with thyroid antibodies had a significantly higher frequency of GADA at the time of the diagnosis. Autoimmune thyroiditis was more prevalent in the older age group. GADA was a significant risk factor for the development of thyroid autoantibodies after the diagnosis of type 1 DM (Odds ratio 4.45, 95% CI 1.399-14.153). Conclusion: In patients with type 1 DM, the prevalence of autoimmune thyroiditis was higher than in the general population. Moreover, GADA positivity at diagnosis was associated with thyroid autoimmunity.