A female, 2 years and 7 months of age, was admitted to the hospital with stupor and nystagmus following projectile vomiting. She had been prenatally diagnosed with trisomy 12p with a familial pericentric inversion of chromosome 12 originating from her mother. She manifested developmental delay and some dysmorphic features of the face and limbs compatible with the clinical features of trisomy 12p. Four-vessel cerebral angiography revealed severe stenosis and occlusion of the supraclinoid portion of the right and left internal carotid arteries with numerous collateral vessels in the vicinity of the occlusion. These features are consistent with moyamoya syndrome. This report presents the first case of moyamoya syndrome with trisomy 12p with a familial pericentric inversion of chromosome 12.