H ypokalemic periodic paralysis (HypoPP) is an ion channelopathy of skeletal muscle characterized by attacks of muscle weakness associated with low serum K⁺. HypoPP results from a transient failure of muscle fiber excitability. Mutations in the genes encoding a calcium channel (Ca_V1.1) and a sodium channel (Na_V1.4) have been identified in HypoPP families. Mutations of Na_V1.4 give rise to a heterogeneous group of muscle disorders, with gain-of-function defects causing myotonia or hyperkalemic periodic paralysis. To address the question of specificity for the allele encoding the Na_V1.4-R669H variant as a cause of HypoPP and to produce a model system in which to characterize functional defects of the mutant channel and susceptibility to paralysis, we generated knockin mice carrying the ortholog of the gene encoding the Na_V1.4-R669H variant (referred to herein as R669H mice). Homozygous R669H mice had a robust HypoPP phenotype, with transient loss of muscle excitability and weakness in low-K⁺ challenge, insensitivity to high-K⁺ challenge, dominant inheritance, and absence of myotonia. Recovery was sensitive to the Na⁺/K⁺-ATPase pump inhibitor ouabain. Affected fibers had an anomalous inward current at hyperpolarized potentials, consistent with the proposal that a leaky gating pore in R669H channels triggers attacks, whereas a reduction in the amplitude of action potentials implies additional loss-of-function changes for the mutant Na_V1.4 channels.