While physical activity (PA) provides established benefits for youth (children and adolescents) development, the differential effects of mentally passive versus mentally active screen time (ST) on academic achievement and social behavior among physically active youth remain unclear. This study examined associations between different types of ST and developmental outcomes in youth who met the PA guideline.Data from the 2016-2017 National Survey of Children's Health were analyzed, including 9967 participants aged 6-17 years who met the PA guideline (≥ 60 min daily) ST was categorized as mentally passive (TV-based) ST and mentally active (computer-based) ST. Academic achievement was assessed through curiosity, psychological resilience, and memory difficulties. Social behaviors consisted of bullying victimization, friendship difficulties, and argumentative behavior. All data were collected through caregiver-completed questionnaires. Multi-variable logistic regression analyses were conducted, controlling for demographic factors including age, sex, ethnicity, socioeconomic status, premature birth and parental education.Mentally passive (TV-based) ST demonstrated predominantly negative associations across all social-cognitive domains. Heavy users (≥4 h daily) showed significantly lower curiosity (OR=0.38, 95 % CI: 0.17-0.89), reduced psychological resilience, nearly four times higher odds of memory difficulties (OR=3.80, 95 % CI: 1.65-8.78), and increased bullying victimization (OR=2.93, 95 % CI: 1.52-5.67). However, mentally active (computer-based) ST showed more complex patterns: light users (<1 h daily) demonstrated higher curiosity and resilience compared to non-users, while heavy users (≥4 h daily) experienced negative outcomes including reduced resilience (OR=0.54, 95 % CI: 0.34-0.87) and increased social difficulties. Both types of ST at high usage levels were associated with increased argumentative behavior (passive ST: OR=2.74, 95 % CI: 1.53-4.91; active ST: OR=2.99, 95 % CI: 1.89-4.74).Among physically active youth, mentally passive and mentally active ST demonstrate distinct associations with academic and social outcomes. Mentally Passive ST shows predominantly negative associations across all domains, while mentally active ST exhibits potential benefits at low usage but negative effects at high usage levels. These findings suggest that ST recommendations should consider both duration and type of sedentary activities, as PA alone may not fully protect against the adverse effects of excessive ST, particularly mentally passive screen exposure.