Introduction: Lymphomatosis cerebri is a rare form of primary central nervous system lymphoma characterized by an atypical clinical presentation and neuroimaging, with a poor short-term prognosis. Case Report: A 51-year-old woman began with clinical manifestations characterized by rapidly progressing cognitive impairment associated with a behavioral disorder, myoclonus, and gait disturbance. The brain magnetic resonance image showed extensive signaling in the bilateral periventricular white matter. The 18F-FDG PET-CT showed severe dorsolateral neocortical hypometabolism in the absence of focal hypermetabolic lesion, a metabolic pattern indicative of cerebri lymphomatosis. A brain biopsy confirmed the diagnosis. The patient started chemotherapy achieving complete remission. Eighteen months after diagnosis, the patient had improved clinically and neuroimaging. Conclusion: This is the first report in Peru of an entity that should be considered in rapidly progressive dementia and leukoencephalopathy cases. Timely diagnosis and appropriate chemotherapy management can increase patient survival.