Angiofollicular lymph node hyperplasia, known more commonly as Castleman disease, is a rare lymphoproliferative disorder. Castleman disease has two distinct clinical manifestations described as unicentric and multicentric disease. These presentations have distinct treatment algorithms and portend very different prognoses. Standard treatment of unicentric disease is complete surgical resection, which confers a cure rate approaching 100%. To our knowledge, this case report is the first to describe the use of neoadjuvant rituximab in the treatment of unicentric Castleman disease to enable a less morbid surgical resection. Given the vascularity of the tumor, proximity to the pulmonary artery and superior vena cava, and possible intimate association with the lung parenchyma, the tumor was treated preoperatively with rituximab, an anti-CD20 monoclonal antibody, at doses of 375 mg/m2 weekly for 4 weeks. Rituximab therapy successfully decreased the diameter of the tumor from 4.79 cm×2.67 cm to 2.8 cm×1.5 cm, as confirmed by CT imaging. Postoperative surgical pathology confirmed the diagnosis of Castleman disease, hyaline vascular type, with negative margins. Notably, the lymph node tissue in the rituximab-treated specimen demonstrated reduced mantle zone thickness, decreased size of follicles, and increased hyalinization of vessels. Rituximab shows promise in neoadjuvant treatment of unresectable or partially resectable unicentric Castleman disease.