Unicentric Castleman disease (UCD) is a rare lymphoproliferative disease that typically occurs in lymph node-rich areas, but primary extranodal forms originating from solid organs outside the lymph nodes are extremely rare. A systematic review analyzed histologically confirmed cases from PubMed over the last 20 years. The median age at diagnosis was 44 years with a male to female ratio of 0.77:1 and a mean tumor size of 4.9 cm. The most common sites were the pancreas (14.1%), adrenal glands (14.1%), skeletal muscle (14.1%), parotid glands (13.1%) and kidneys (9.1%). The hyaline vascular type prevailed (80.8%). Surgical resection was the primary treatment with a 4-year recurrence-free survival rate of 84.6% (95% CI, 0.512–0.959); transformation to lymphoma occurred in 2% of cases. Radiotherapy and glucocorticoids may be effective options. Primary extranodal UCD has an excellent prognosis after complete surgical resection, but requires long-term management due to the risk of malignant transformation.