Progressive multifocal leukoencephalopathy (PML) is a rare fatal central nervous system disorder characterized by infection-induced demyelination of white matter due to the opportunistic reactivation of John Cunningham virus in an immunocompromised patient. PML is associated with many immune-mediated diseases, lymphoproliferative conditions, and immunosuppressive agents. In this case report, we present a 79-year-old female patient diagnosed with rheumatoid arthritis who developed posterior fossa PML while on rituximab. She presented with subacute cerebellar ataxia, dysarthria, and nystagmus, and her brain MRI showed right pontine and pontocerebellar lesion with diffusion restriction and heterogenous enhancement highly characteristic of PML. Though many cases of PML with rituximab were reported in the literature, our case describes a rare type of PML affecting the posterior fossa in an HIV-negative patient on rituximab.