Diffuse leptomeningeal glioneuronal tumor is characterized by hydrocephalus, leptomeningeal involvement in the absence of a primary parenchymal mass, and negative cerebrospinal fluid (CSF) cytology. It is an extremely rare and difficult tumor to diagnose as no mass can be biopsied and it mimics infectious, rheumatologic, and inflammatory pathologies. An 11-year-old girl presented with complaints of headache, vomiting, and double vision. On examination, there was papilledema. Initial MRI scanning did not yield any significant findings. Clinical progression was observed in four months in the follow-ups. The symptoms included seizures, gait disturbances, and severely increased intracranial pressure. The screening of the patient for infectious, rheumatologic, endocrinologic, and inflammatory pathologies was normal. CSF pressure was elevated without any malignancy. Repeated cranial MRI revealed hydrocephalus and pituitary expansion. Leptomeningeal thickening and contrast enhancement were observed in spinal MRI. After a negative dural biopsy, the patient was diagnosed with a spinal leptomeningeal biopsy. The authors believed that the prevalence of this rare pediatric tumor, diagnosed with a leptomeningeal biopsy, is underestimated as it has an insidious course and signs of increased intracranial pressure in the absence of a definite solid mass.