Sodium channels initiate the electrical cascade responsible for cardiac rhythm, and certain life-threatening arrhythmias arise from Na⁺ channel dysfunction. We propose a novel mechanism for modulation of Na⁺ channel function whereby calcium ions bind directly to the human cardiac Na⁺ channel (hH1) via an EF-hand motif in the C-terminal domain. A functional role for Ca²⁺ binding was identified electrophysiologically, by measuring Ca²⁺-induced modulation of hH1. A small hH1 fragment containing the EF-hand motif was shown to form a structured domain and to bind Ca²⁺ with affinity characteristic of calcium sensor proteins. Mutations in this domain reduce Ca²⁺ affinity in vitro and the inactivation gating effects of Ca²⁺ in electrophysiology experiments. These studies reveal the molecular basis for certain forms of long QT syndrome and other arrhythmia-producing syndromes, and suggest a potential pharmacological target for antiarrhythmic drug design.