["Journal of School Health, Volume 96, Issue 5, May 2026. ", "\nABSTRACT\n\nBackground\nTo better understand the key behavioral factors for adolescent obesity, this study investigated the independent and combined associations of physical activity (PA) and sedentary time (ST) with obesity prevalence across different school grades.\n\nMethods\nCross‐sectional data from 614,943 adolescents in the Korea Youth Risk Behavior Web‐based Survey were analyzed. Independent associations of PA and ST with obesity were examined based on weekly PA frequency and daily nonacademic ST. Combined associations were assessed using adherence criteria (≥ 60 min/day of moderate‐intensity PA plus ≥ 3 days/week of vigorous‐intensity PA) and ST classification (≤ 2 h/day).\n\nResults\nIn independent analysis, adolescents meeting PA guidelines consistently showed lower odds of obesity across all grades. For sedentary time, the ≤ 2 h/day group had the lowest odds, and a dose–response relationship was observed up to 6–8 h/day across grades. In combined analysis, adolescents meeting both PA and ST guidelines had the lowest obesity prevalence (OR = 0.54, 95% CI: 0.50–0.59).\n\nImplications for School Health Policy, Practice, and Equity\nSchools should promote daily PA and reduce ST. Since behavioral risk factors differ by developmental stage (e.g., academic stress in higher grades), grade‐tailored strategies are essential for effective obesity prevention.\n\nConclusions\nHigher PA levels and lower ST are independently and jointly associated with lower obesity prevalence among Korean adolescents across all school grades.\n"]