An atypical teratoid/rhabdoid tumor (AT/RT) is a highly malignant embryonal tumor most often occurring in the posterior fossa in children younger than 3 years of age. Adult cases of AT/RT are very rare, and 27 cases with a diagnosis of either AT/RT or (malignant) rhabdoid tumor have been reported to date. The authors report an adult case of an AT/RT occurring in the pineal region with molecular cytogenetic and immunohistochemical confirmation. A 33-year-old woman presented with a 2-month history of headache and blurred vision progressing to diplopia, and was admitted emergently due to deteriorating mental status. An MR image showed a heterogeneously enhancing mass involving the posterior third ventricle and pineal region with mild hydrocephalus. She underwent a subtotal resection of the tumor and was then treated with chemoradiation. Thirteen months after surgery, she was still alive with radiological evidence of recurrence/residual lesions. Histological sections showed epithelioid cellular sheets of rhabdoid tumor cells with scattered mitotic figures. Immunohistochemically, the tumor cells were diffusely and strongly positive for epithelial membrane antigen and vimentin, and showed focal expression of glial fibrillary acidic protein, pancytokeratin, and neurofilament protein. Loss of nuclear immunoreactivity for INI1 protein was observed. Fluorescence in situ hybridization analysis showed monosomy 22. Histologically, this tumor consisted exclusively of epithelioid tumor cells with rhabdoid features. The differential diagnoses include rhabdoid glioblastoma, metastatic carcinoma, and rhabdoid meningioma. Molecular testing to identify monosomy 22 or deletions of the chromosome 22q11 containing the INI1/hSNF5 gene and/or immunohistochemical staining with INI1 antibody is of great importance for the diagnosis of this tumor.