A male child, aged 5 years and 3 months, was admitted to the Oncology Department with a history of pain in both hip joints, headache, and diplopia lasting for 40 days. Physical examination did not reveal definitive signs or obvious abnormalities in the nervous system. Imaging studies showed only abnormalities in the craniocerebrum and spinal cord. Routine cerebrospinal fluid (CSF) analysis revealed elevation in the total number of white blood cells, mainly mononuclear cells. Biochemical analysis of CSF showed normal glucose and chloride levels, and increased protein concentrations. The possibility of central nervous system (CNS) infection was initially considered. Subsequently, antibacterial and antiviral therapy was administered; however, this treatment was ineffective. Further examination of CSF through immunophenotyping revealed mature B-cell lymphoma with CNS involvement; there were no neoplastic lesions detected elsewhere in the body. Thus, the patient was diagnosed with primary central nervous system lymphoma (PCNSL). Complete remission was achieved after chemotherapy with the CNCL-2017-mature B-cell lymphoma regimen. Thus far, all chemotherapy cycles have been completed, the patient remains in complete remission, and the follow-up is ongoing. Clinicians should pay close attention to PCNSL in children.