Introduction: Primary Central Nervous System Lymphoma (PCNSL) is a rare malignancy primarily located in the brain, spinal cord, leptomeninges, and eyes. The origin of PCNSL is uncertain, but it is thought to arise from lymphocytes within the central nervous system. Diagnosis and treatment can be complex, especially in immunocompetent patients. Case Report: A 65-year-old immunocompetent woman presented with symptoms including weight loss, severe weakness, and left hemiplegia. Initial brain MRI revealed an irregular mass with vasogenic edema in the right parietal lobe. Stereotactic biopsy confirmed large cell lymphoma (T-cell variant). The patient initially responded well to high-dose steroid treatment. Subsequent chemotherapy and radiotherapy were planned, but due to a positive PCR test, treatment was delayed. Discussion: PCNSL presents with non-specific symptoms, making early diagnosis challenging. Diagnostic tools such as MRI, PET-CT, and immunohistochemistry play crucial roles. Steroid use before biopsy should be cautious, as it may affect tissue necrosis and diagnosis accuracy. Prompt diagnostic and therapeutic measures are essential for improved outcomes. Conclusion: PCNSL is a rare and aggressive malignancy with a poor prognosis if left untreated. Early diagnosis and appropriate treatment, including chemotherapy and radiotherapy, are crucial for patient survival. Larger studies are needed to further understand and manage this challenging condition.