Cerebrovascular dysfunction is strongly associated with neonatal intracranial hemorrhage (ICH) and stroke in adults. Cerebrovascular endothelial cells (ECs) play important roles in maintaining a stable cerebral circulation in the central nervous system by interacting with pericytes. However, the genetic mechanisms controlling the functions of cerebral ECs are still largely unknown. Here, we report that disruption of Smad4, the central intracellular mediator of transforming growth factor-β (TGF-β) signaling, specifically in the cerebral ECs, results in perinatal ICH and blood-brain barrier breakdown. Furthermore, the mutant vessels exhibit defective mural cell coverage. Smad4 stabilizes cerebrovascular EC-pericyte interactions by regulating the transcription of N-cadherin through associating with the Notch intracellular complex at the RBP-J binding site of the N-cadherin promoter. These findings uncover a distinct role of endothelial Smad4 in maintaining cerebrovascular integrity and suggest important implications for genetic or functional deficiencies in TGF-β/Smad signaling in the pathogenesis of cerebrovascular dysfunction.