Objective To improve clinician's understanding of the clinicopathological and imaging features of primary central nervous system malignant melanoma. Methods The clinical diagnosis and treatment data of one patient with primary central nervous system malignant melanoma were retrospectively analyzed, including clinical symptoms, magnetic resonance imaging (MRI) and PET-CT findings, as well as cerebrospinal fluid cytology and brain biopsy pathology. The diagnostic and therapeutic approaches were discussed in conjunction with relevant literature. Results The patient in this paper was an elderly male presenting with rapidly progressive dementia and gait imbalance as the main clinical manifestations. Cranial MRI demonstrated hyperintensities on T1- and T2-weighted imaging with significant contrast enhancement in extensive cortical involvement, lesions in the right cerebellar hemisphere, and bilateral sulcal, along with supratentorial hydrocephalus. Cerebrospinal fluid cytology revealed the presence of atypical cells containing melanin granules, and the diagnosis of primary central nervous system malignant melanoma was finally confirmed through pathological examination of a brain biopsy. Conclusion Patients with primary central nervous system malignant melanoma, characterized by diffuse leptomeningeal involvement and hydrocephalus on imaging, may present with rapidly progressive dementia. The characteristic MRI findings and cerebrospinal fluid cytological changes serve as key clues for early diagnosis, while definitive confirmation still relies on pathological examination.