A 48-year-old female presented with a rare case of adult T-cell leukemia/lymphoma (ATL) occurring as only intracerebral mass lesion manifesting as progressively worsening headaches, transient mild weakness of the left lower extremity, bilateral papilledema, and left homonymous hemianopsia. Laboratory examination showed transient leukocytosis (15900/μl) without febrile episode or elevation of C-reactive protein. Neuroimaging revealed a solitary enhanced mass lesion in the right occipital lobe adjacent to the choroid plexus with prominent perifocal edema. The patient underwent gross total removal, and the histological diagnosis was intracerebral ATL. She underwent radiation therapy and chemotherapy after local recurrence and metastasis to an optic nerve. The lesions had disappeared on magnetic resonance imaging with contrast medium 10 months after onset and no recurrence was detected even 5 years later. Intracerebral ATL should be considered in the differential diagnosis of intracerebral mass without leukemia or systemic lymphoma.