Neurogenesis occurs throughout the life in the mammalian brain. The hippocampal dentate gyrus (DG) is one of the major regions of the adult neurogenesis, where neural stem/progenitor cells continuously generate new granule neurons, although molecular mechanisms underlying generation and maintenance of newly born neurons are still elusive. Here we show that ephrin-A5, a ligand for Eph receptor tyrosine kinases, plays multiple roles in both neurogenesis and vascular formation in the adult hippocampus. In mice lacking ephrin-A5 function, cell proliferation and survival of newborn neurons were severely reduced in the hippocampus DG. Furthermore, ephrin-A5-deficient mice exhibited altered distribution of EphA4 receptor in the vascular endothelial cells and increased narrower capillaries in the hippocampus DG. EphA/ephrin-A signaling thus plays crucial roles in the establishment and/or maintenance of the brain vascular system, as an essential constituent of the adult neurogenic niche.