Previous studies have demonstrated that the appropriate production of serum anti-severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2) neutralizing antibody (nAb) plays a critical role in the recovery from coronavirus disease 2019 (COVID-19); however, the role of nAb production in the recovery from a flare-up of chronic immune thrombocytopenia (ITP) has been unknown. We here report the first retrospectively investigated case of serum anti-SARS-Cov-2 nAb production during chronic ITP flare-up triggered by COVID-19. A 79-year-old woman with a history of corticosteroid-refractory ITP visited our hospital complaining of fever, cough, and sore throat for 4 days. Although chronic ITP was controlled by 12.5 mg of eltrombopag (EPAG) every other day, laboratory tests showed a decreased peripheral blood platelet count of 15.0 × 109/L, which indicated worsening thrombocytopenia. Meanwhile, PCR testing of a nasopharyngeal swab revealed that the patient was positive for SARS-Cov-2, and a computed tomography scan revealed bilateral pneumonia. On the basis of the flare-up of chronic ITP associated with COVID-19 pneumonia which was determined as a moderately severe status according to the WHO clinical progression scale, intravenous immunoglobulin therapy for 5 days (days 0-4) and antiviral therapy were added on top of EPAG, which only resulted in a transient increase in the platelet count for several days. After decreasing to 8.0 × 109/L on day 13, the platelet count increased from day 16, coinciding with a positive detection for serum nAb against SARS-Cov-2. Although the increased dose up to 50 mg/day of EPAG was challenged during the clinical course, rapid dose reduction did not cause another relapse. In addition, no thrombotic or bleeding event was seen. These collectively suggest the vital role of the production of anti-SARS-Cov-2 nAb and improvement of clinical symptoms for recovery from a flare-up of chronic ITP in our case.