Brugada syndrome (BrS) is a known cause of sudden cardiac death (SCD) characterized by abnormal electrocardiograms and fatal arrhythmias. The variants inKCND3encoding the K(V)4.3 potassium-channel (the alpha-subunit of the I-to) have seldom been reported in BrS. This study aimed to identify novelKCND3variants associated with BrS and elucidate BrS pathogenesis. High-depth targeted sequencing was performed and the electrophysiological properties of the variants were detected by whole-cell patch-clamp methods in a cultured-cell expressing system. The transcriptional levels of K(V)4.3 in different genotypes were studied by real-time PCR. Western blot was used to assess channel protein expression. A novelKCND3heterozygous variant, c.1292G>A (Arg431His, R431H), was found in the proband. Whole-cell patch-clamp results revealed a gain-of-function phenotype in the variant, with peak I-to current density increased and faster recovery from inactivation. The expression of mutant Kv4.3 membrane protein increased and the cytoplasmic protein decreased, demonstrating that the membrane/cytoplasm ratio was significantly different. In conclusion, a novelKCND3heterozygous variant was associated with BrS. The increased I-to current explained the critical role ofKCND3in the pathogenesis of BrS. Genetic screening forKCND3could be useful for understanding the pathogenesis of BrS and providing effective risk stratification in the clinic.