Abstract: The most common etiologies of subacute cerebellar ataxia include atypical infections, autoimmune disorders, and paraneoplastic cerebellar degeneration. We reported a rare etiology of subacute cerebellar ataxia in a young man. A 31-year-old man was admitted to our hospital for subacute progressive cerebellar ataxia. He was detected HIV positive, and DNA of JC virus positive in his cerebrospinal fluid. He was diagnosed with JC virus cerebellar granule cell neuronopathy, and symptoms got improved after anti-retroviral therapy. At the 3-month follow up, his condition was stable. Cerebellar granule cell neuronopathy is a novel syndrome caused by JC virus infection. Our case indicated that JC virus infection should be considered when an immune-compromised patient with positive HIV develops cerebellar ataxia. Restoring immune function helps to control the progress of JC virus cerebellar granule neuronopathy. We summarized the diagnostic approach and treatment of subacute cerebellar ataxia through retrospectively reviewing the clinical data of the young man.