During September 2023-August 2024, approximately 38,000 COVID-19-associated hospitalizations occurred among children and adolescents aged <18 years in the United States, a rate of approximately 53 per 100,000 children, ranging from 600 per 100,000 children aged <6 months to 21 per 100,000 children and adolescents aged 5-17 years. On June 27, 2024, the Advisory Committee on Immunization Practices recommended that all persons aged ≥ 6 months receive a 2024-2025 COVID-19 vaccine, which targeted Omicron JN.1 and JN.1-derived sublineages. Investigators used a test-negative case-control design to estimate vaccine effectiveness (VE) of 2024-2025 COVID-19 vaccines against COVID-19-associated emergency department or urgent care (ED/UC) visits during August 29, 2024-September 2, 2025, among immunocompetent children aged 9 months-4 years and children and adolescents aged 5-17 years in the CDC-funded Virtual SARS-CoV-2, Influenza, and Other respiratory viruses Network (VISION), a multisite electronic health record-based network in nine states. Among children aged 9 months-4 years, VE against COVID-19-associated ED/UC visits was estimated at 76% (95% CI = 58%-87%) during the first 7-179 days after vaccination. Among children and adolescents aged 5-17 years, VE against COVID-19-associated ED/UC visits was an estimated 56% (95% CI = 35%-70%) during the first 7-179 days after vaccination. These findings suggest that vaccination with a 2024-2025 COVID-19 vaccine dose provided children with additional protection against COVID-19-associated ED/UC encounters compared with no 2024-2025 dose.