T-type Ca²⁺ channels (T-channels) are involved in the control of neuronal excitability and their gating can be modulated by a variety of redox agents. Ascorbate is an endogenous redox agent that can function as both an anti- and pro-oxidant. Here, we show that ascorbate selectively inhibits native Ca_v3.2 T-channels in peripheral and central neurons, as well as recombinant Ca_v3.2 channels heterologously expressed in human embryonic kidney 293 cells, by initiating the metal-catalyzed oxidation of a specific, metal-binding histidine residue in domain 1 of the channel. Our biophysical experiments indicate that ascorbate reduces the availability of Ca_v3.2 channels over a wide range of membrane potentials, and inhibits Ca_v3.2-dependent low-threshold-Ca²⁺ spikes as well as burst-firing in reticular thalamic neurons at physiologically relevant concentrations. This study represents the first mechanistic demonstration of ion channel modulation by ascorbate, and suggests that ascorbate may function as an endogenous modulator of neuronal excitability.