Signal Transducer and Activator of Transcription 1 (STAT1) is a DNA-binding signal transducer that regulates transcription of specific genes in response to IFNγ and IFNα/β stimulation. Loss-of-function mutations impairing STAT1 activity are known to confer susceptibility to intracellular bacterial and viral diseases. Conversely, the few known activating mutations of STAT1 allow predisposition to chronic mucocutaneous candidiasis disease, and occur in patients with combined immunodeficiency and defective Th1 and Th17 responses. Here, we report on a de novo gain-of-function (GoF) STAT1 mutation (c.1398C>G, p.Ser466Arg) identified by exome sequencing in an individual with brain calcification, arthritis, recurrent pericarditis, leukopenia, thrombocytopenia and low C3 levels, a phenotype resembling an interferonopathy. The Ser466Arg change affects a highly conserved residue located in the DNA binding domain of the protein and the amino acid substitution was documented to have an activating role both in vitro and in vivo. Altogether, clinical features and functional studies are compatible with hyperactivation of the Interferon pathways, highlighting a role of STAT1 GoF mutation in clinical phenotypes fitting interferonopathies.