Coronavirus disease 2019 (COVID-19) is affecting mental health profoundly. Previous studies have reported pandemic-related anxiety. Anxiety disorder and autism spectrum disorder (ASD) are common comorbidities. However, there has been no report of any patient with undiagnosed ASD who developed anxiety disorders caused by the COVID-19 pandemic. In this case report, we describe an 8-year-old Japanese boy with undiagnosed ASD who developed COVID-19 phobia, resulting in avoidant restrictive food intake disorder (ARFID).