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Postpartum thyroiditis (PPT) is characterized by mild thyrotoxicosis occurring within one year of parturition commonly followed by transient hypothyroidism. Having genetic background of autoimmune thyroid disorders is a risk factor for it because the immune reactivation during postpartum period is a trigger for PPT. Pandemic of COVID-19: caused by SARS-CoV-2 infection is a global health problem, and occurrence of Graves' disease and Hashimoto's thyroiditis after the viral infection have been reported but occurrence of PPT with COVID-19 has never been reported. A 29-year-old woman developed general fatigue four and a half months after parturition, and was diagnosed as having PPT: one month before, she had COVID-19. Hereafter, we define the date of delivery as Day 0 to make timeline clear. SARS-CoV-2 infection was diagnosed by PCR on Day 103, its disappearance from the upper airway confirmed on Day 124, and the thyroiditis diagnosed on Day 136. She had been euthyroid on Day 0 and 95, but thyrotoxic on Day 136. Serum thyroglobulin (Tg) concentration was normal in the presence of anti-Tg antibody, other thyroid-related autoantibodies were negative, and by ultrasonography, the thyroid gland was normal in size and no evidence of increased vascularity. Thyroid function returned to normal by Day 172 without any specific drug therapy. In conclusion, although a clear causal relationship could not be found, we documented the world's first case of PPT developed following COVID-19.