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Long QT Syndrome (LQTS) is a rare inherited cardiac disorder with a high risk of sudden cardiac death in a structurally normal heart. To date, pathogenic mutations have been described as responsible for approximately 70-75% of LQTS patients, mainly in ion channel genes. Mutations in genes encoding the sodium channel α subunit or other regulatory proteins, affecting cardiac sodium current, have been previously related to LQTS. Five sodium channel β subunits have been identified, which are encoded by four genes (SCN1B-4B). Pathogenic mutations in the SCN4B gene, but not in other β subunits, have been reported in LQTS. We tested whether mutations in SCN1B-4B could be responsible for LQTS in patients without mutations in the common LQTS-related genes. We screened for mutations in SCN1B-4B genes in 30 non-related patients clinically diagnosed with LQTS carrying no mutations in the major LQTS-related genes. The screening revealed a novel mutation in the SCN1B gene in an 8-year-old boy. The base change resulted in an amino acid variation from proline to threonine in the alternative C-terminus of the sodium channel β1 subunit (β1b). Using the patch clamp technique, we measured sodium current density, and Nav1.5 gaiting properties, in HEK cells transiently transfected with Nav1.5 and β1b subunits. Our electrophysiological analysis revealed that the mutant β1b altered Nav1.5 function by shifting the window current to negative potentials, increasing recovery from inactivation, decreasing slow inactivation, and increasing late sodium current. In addition, we recorded action potentials from mouse atrial cardiomyocytes, HL-1 cells, transfected with β1b subunits. These experiments revealed that the action potential duration significantly increased when the mutant β1b was overexpressed compared to β1bWT. These findings suggest that the mutation in β1b could explain the LQTS in our patient, revealing SCN1Bb as a new susceptibility gene for LQTS. Our results confirm the importance of sodium channel β subunits in the modulation of cardiac sodium channel. In addition, they highlight the need for further investigation to detect new candidate genes underlying the LQTS patients that currently remain without genetic diagnosis.