Neurotrophic factors direct the development of the nervous system, and impairments in their function lead to neurological disorders. Leptin is an adipokine that exerts neurotrophic effects in the hippocampus, promoting synaptogenesis and synaptic plasticity. Meanwhile, rodents with altered leptin signaling exhibit impairments in these processes and defects in hippocampal-related functions. Leptin increases mushroom spines and the frequency of mini excitatory postsynaptic currents (mEPSCs) in developing hippocampal neurons, evidencing increased functional glutamatergic synapses. Given the presumptive role of dendritic spines on learning and memory, leptin's role in spinogenesis may explain the defects observed in rodents with impaired leptin signaling. Nonetheless, the molecular mechanisms underlying leptin neurotrophic effects are poorly understood.The endopeptidase matrix metalloproteinase 9 (MMP9), involved in extracellular matrix processing, is essential for synaptic plasticity in the hippocampus. Interestingly, leptin induces MMP9 expression and activity in non-neural cells; however, it is unknown whether MMP9 plays a role in leptin neurotrophic effects on hippocampal neurons. Using a combination of comprehensive approaches, we investigated the role of MMP9 in the effects of leptin on dendritic spines in the developing hippocampus. We found that leptin increased MMP9 expression, release, and activity in cultured hippocampal neurons. Additionally, leptin failed to increase mushroom spines and mEPSCs in MMP9 KO mice. Finally, blocking cathepsin B expression also prevented leptin effects on dendritic spines. Thus, our results indicate that MMP9 and cathepsin B are necessary for leptin effects on the structural plasticity of dendritic spines in the developing hippocampus and that leptin regulates MMP9 activity in neurons.