Objective: Riedel thyroiditis (RT) is a rare disorder with high morbidity and limited treatment options. We describe a case resistant to conventional treatment with corticosteroids and tamoxifen, which subsequently responded to rituximab. Methods: Surgical pathology with hematoxylin and eosin staining was initially performed to confirm diagnosis, followed by IgG4 immunostaining and IgG4 serology in the setting of refractory RT, given its association with IgG4-related disease. Response to treatment was monitored subjectively as well as objectively with serial computed tomography scans. Results: A 51-year-old female with history of Hashimoto thyroiditis presented with compressive neck symptoms. Imaging was suggestive of a multinodular goiter with a large right thyroid nodule. Total thyroidectomy was planned, however intraoperative findings of a densely adherent thyroid with disruption of resection planes led to early termination of surgery. Biopsies obtained during surgery showed benign thyroid tissue with chronic lymphocytic thyroiditis, dense fibrous scar tissue, and benign lymph nodes, confirming the diagnosis of RT. The patient had minimal symptomatic improvement with chronic prednisone as high as 60 mg daily with tamoxifen at 30 mg twice a day. She subsequently received 4 doses of intravenous rituximab at 375 mg/m 2 every 3 weeks, resulting in significant subjective improvement of her compressive symptoms as well as an objective decrease in size of the thyroid mass as seen on a subsequent computed tomography scan. Conclusion: Evidence regarding etiology and management of RT is limited. We present a case of refractory RT treated with rituximab with resultant symptomatic improvement, thus providing further evidence for use of rituximab in resistant cases. (AACE Clinical Case Rep. 2019;5:e218-e221) Abbreviations: IgG4-RD = IgG4-related disease; RT = Riedel thyroiditis