In leptin-deficient ob/ob mice, obesity and diabetes are associated with abnormal development of neurocircuits in the hypothalamic arcuate nucleus (ARC), a critical brain area for energy and glucose homoeostasis^,. Because this developmental defect can be remedied by systemic leptin administration, but only if given before postnatal day 28, a critical period for leptin-dependent development of ARC neurocircuits has been proposed. In other brain areas, critical-period closure coincides with the appearance of perineuronal nets (PNNs), extracellular matrix specializations that restrict the plasticity of neurons that they enmesh. Here we report that in humans and rodents, subsets of neurons in the mediobasal aspect of the ARC are enmeshed in PNN-like structures. In mice, these neurons are densely packed into a continuous ring that encircles the junction of the ARC and median eminence, which facilitates exposure of ARC neurons to the circulation. Most of the enmeshed neurons are both γ-aminobutyric acid-ergic and leptin-receptor positive, including a majority of Agouti-related-peptide neurons. Postnatal formation of the PNN-like structures coincides precisely with closure of the critical period for maturation of Agouti-related-peptide neurons and is dependent on input from circulating leptin, because postnatal ob/ob mice have reduced ARC PNN-like material that is restored by leptin administration during the critical period. We conclude that neurons crucial to metabolic homoeostasis are enmeshed in PNN-like structures and organized into a densely packed cluster situated circumferentially at the ARC–median eminence junction, where metabolically relevant humoral signals are sensed.