Primary angiitis of the central nervous system (PACNS) is one of the most devastating pathologic processes that affect the central nervous system (CNS). It results in exclusive inflammation and destruction of CNS blood vessels. Progressive debilitating unexplained neurological deficit associated with abnormal cerebrospinal fluid (CSF) analysis findings is the typical picture of the disease. CNS biopsy is the gold standard diagnostic test. Immunosuppressive therapy is the core treatment. Reversible cerebral vasoconstriction syndrome (RCVS) is a main mimic of PACNS. RCVS is characterized clinically by recurrent thunderclap headache with or without neurological deficit and normal CSF analysis findings and angiographically by reversible diffuse segmental vasospasm of intracranial vessels. A stepwise diagnostic approach should be followed to differentiate PACNS from RCVS and exclude the other clinical, radiographic, and angiographic mimics.