Early-onset atrial fibrillation (AF) can be the manifestation of a genetic atrial myopathy. However, specific genetic identification of a mutation causing atrial fibrosis is rare. We report a case of a young patient with an asymptomatic AF, diagnosed during a routine examination. The cardiac MRI revealed extensive atrial fibrosis and the electrophysiology study showed extensive areas of low voltage. The genetic investigation identified a homozygous pathogenic variant in the NPPA gene in the index case and the presence of the variant in heterozygosity in both parents.