A 13-year-old girl presented to our hospital with chief complaints of rapid weight loss, fatigue, discomfort, chills in the extremities, and alopecia. We initially suspected anorexia nervosa (AN). However, she did not express fear of gaining weight or have a distorted perception of her weight or body shape; thus, her presentation was not typical of AN. We also suspected avoidant/restrictive food intake disorder (ARFID), but she did not exhibit any food-avoidance behaviors. However, she was obsessed with nutrition control, so we diagnosed her with orthorexia nervosa (ON). She was hospitalized, given education on proper nutrition, and her eating behavior subsequently improved. After discharge, we administered the ORTO-15, which assesses the propensity for ON, and her score met the diagnostic criteria for ON. The incidence of ON has increased during the COVID-19 pandemic. In this case, her obsession was brought about by information she read in magazines and on social media that promoted an unbalanced diet centered almost exclusively on vegetables. Pediatricians should raise awareness of misinformation regarding children's health to ensure healthy growth.