During the coronavirus disease 2019 (COVID-19) outbreak, the prevalence of obesity increased globally; this may be associated with hypertension incidence. However, investigations on the changes in the prevalence of hypertension among children and adolescents are limited. This cross-sectional study investigated the prevalence of hypertension among 1428 youths aged 10–18 years using data from the Korea National Health and Nutrition Examination Survey 2018–2020. We assessed the prevalence of hypertension according to sex, age, body mass index (BMI), and residential district. The prevalence of hypertension increased from 7.1% to 12.5% in all participants. In the sex-specific analysis, the prevalence was found to be increased in boys. In the age-specific analysis, the prevalence was found to be increased in youths aged 13–15 years. In the BMI-specific analysis, an increase in the prevalence was prominent in the normal BMI group. In the residential district-specific analysis, the prevalence of hypertension among youth increased in urban areas. Our results show that the prevalence of hypertension increased among Korean children and adolescents during the COVID-19 outbreak. These findings suggest the importance of close monitoring of hypertension among youth during the COVID-19 pandemic.