Pediatric inflammatory bowel disease is associated with growth failure due to chronic inflammation, nutrient disorder, and the side effects of drugs, such as corticosteroids. Biological agents are therapeutic drugs that significantly improve the prognosis of patients with inflammatory bowel disease. The effectiveness of ustekinumab has been reported in the management of adult patients with inflammatory bowel disease. There are very few reports regarding the effectiveness and safety of ustekinumab in pediatric patients with inflammatory bowel disease, especially those who are biologically naive. A 10-year-old girl presented with chronic abdominal pain, diarrhea, and weight loss. Colonoscopy showed a longitudinal ulcer and cobblestone appearance in the ileum and discontinuous inflammation of the colon; therefore, she was diagnosed with Crohn's disease. She was prescribed a fat-restricted diet, elemental diet, 5-aminosalicylic acid, transient prednisolone, and ustekinumab. She achieved clinical and endoscopic remission based on the weighted Pediatric Crohn's Disease Activity Index, fecal calprotectin, and colonoscopy findings at week 75. This patient developed no adverse events, such as infusion reaction or susceptibility to infection over the 75 weeks. The use of ustekinumab as the first biological agent may be an effective and safe treatment for pediatric Crohn's disease.