AbstractLittle is known about the properties and function of ion channels that affect synaptic terminal-resting properties. One particular subthreshold-active ion channel, the Kv7 potassium channel, is highly localized to axons, but its role in regulating synaptic terminal intrinsic excitability and release is largely unexplored. Using electrophysiological recordings together with computational modeling, we found that the KV7 current was active at rest in adult hippocampal mossy fiber synaptic terminals and enhanced their membrane conductance. The current also restrained action potential-induced Ca2+influx via N- and P/Q-type Ca2+channels in boutons. This was associated with a substantial reduction in the spike half-width and afterdepolarization following presynaptic spikes. Further, by constraining spike-induced Ca2+influx, the presynaptic KV7 current decreased neurotransmission onto CA3 pyramidal neurons and short-term synaptic plasticity at the mossy fiber–CA3 synapse. This is a distinctive mechanism by which KV7 channels influence hippocampal neuronal excitability and synaptic plasticity.