Atrial fibrillation (AF) is the most common form of sustained clinical arrhythmia. We previously mapped an AF locus to chromosome 5p13 in an AF family with sudden death in early childhood. Here we show that the specific AF gene underlying this linkage is NUP155, which encodes a member of the nucleoporins, the components of the nuclear pore complex (NPC). We have identified a homozygous mutation, R391H, in NUP155 that cosegregates with AF, affects nuclear localization of NUP155, and reduces nuclear envelope permeability. Homozygous NUP155(-/-) knockout mice die before E8.5, but heterozygous NUP155(+/-) mice show the AF phenotype. The R391H mutation and reduction of NUP155 are associated with inhibition of both export of Hsp70 mRNA and nuclear import of Hsp70 protein. These human and mouse studies indicate that loss of NUP155 function causes AF by altering mRNA and protein transport and link the NPC to cardiovascular disease.