Paediatric patients with the syndrome of an inappropriate antidiuretic hormone secretion (SIADH), as a manifestation of inflammatory demyelinating disorders of the central nervous system, have been rarely described until now, in only a few cases of neuromyelitis optica spectrum disorders (NMOSDs). We present a case of relapsing SIADH associated with NMOSD, in an anti-aquaporin-4 antibody positive 14-year-old girl, who is, to our best knowledge, the first reported paediatric patient with relapsing SIADH and NMOSD. Additionally, our case further supports the notion that paediatric encephalomyelitis associated with SIADH should suggest the diagnosis of NMOSD.