Physical exercise stimulates adult hippocampal neurogenesis and is considered a relevant strategy for preventing age-related cognitive decline in humans. The underlying mechanisms remains controversial. Here, we show that exercise increases proliferation of neural precursor cells (NPCs) of the mouse dentate gyrus (DG) via downregulation of microRNA 135a-5p (miR-135a). MiR-135a inhibition stimulates NPC proliferation leading to increased neurogenesis, but not astrogliogenesis, in DG of resting mice, and intriguingly it re-activates NPC proliferation in aged mice. We identify 17 proteins (11 putative targets) modulated by miR-135 in NPCs. Of note, inositol 1,4,5-trisphosphate (IP3) receptor 1 and inositol polyphosphate-4-phosphatase type I are among the modulated proteins, suggesting that IP3 signaling may act downstream miR-135. miR-135 is the first noncoding RNA essential modulator of the brain's response to physical exercise. Prospectively, the miR-135-IP3 axis might represent a novel target of therapeutic intervention to prevent pathological brain aging. Pons-Espinal, Gasperini, and colleagues report that running induces NPC proliferation and neurogenesis via downregulation of miR-135a-5p in the mouse hippocampus. Remarkably, downregulation of miR-135a stimulates proliferation in the hippocampus of aged mice. ITPR1 and INPP4A, involved in IP3 signaling, are modulated by miR-135 in NPCs. Prospectively, therapeutic exploitation of the miR-135-IP3 axis might represent a novel intervention strategy for successful aging.