COPA syndrome is a newly discovered hereditary immunodeficiency affecting the lung, kidneys, and joints. The mutated gene encodes the α subunit of the coatomer complex I, a protein transporter from the Golgi back to the endoplasmic reticulum. The impaired return of proteins leads to intracellular stress. The syndrome is an autoimmune and autoinflammatory disease that can be grouped among the interferonopathies. The knowledge about COPA syndrome and its treatment is still limited. In this paper, we describe an additional patient, a 15-year-old girl with rheumatoid factor-positive polyarthritis and rheumatoid nodules since the age of 2, who developed interstitial lung disease. The detected mutation c.698G>A was causing the disease. The patient presented with symmetric polyarthritis on wrists, fingers, and hip and ankle joints, with significant functional impairment, and high disease activity. Laboratory parameters demonstrated chronic inflammation, hypergamma-globulinemia, high titre ANA (antinuclear antibodies) and CCP (anti-citrullinated protein) antibodies, and rheumatoid factors. Therapies with various DMARDs (Disease Modifying Anti-Rheumatic Drugs) and biologicals failed. Upon baricitinib application, the clinical activity decreased dramatically with disappearance of joint pain and morning stiffness and significant decrease of joint swelling. A low disease activity was reached after 12 months, with complete disappearance of rheumatoid nodules. In contrast to IL-1 (interleukin-1), IL-6, and TNF (tumor necrosis factor) inhibitors, baricitinib was very successful, probably because baricitinib acts as a JAK-1/2 (janus kinase-1/2) inhibitor in the IFNα/β (inteferone α/β) pathway. A relatively higher dose in children is necessary. COPA syndrome represents a novel disorder of intracellular transport. Reviewing published literature on COPA syndrome, in addition to our patient, there were 31 cases further described.