Physical activity (PA) of sufficient intensity to improve aerobic fitness is associated with enhanced executive functions (EFs) in preadolescents. However, the relationship between PA intensity and EFs among older adolescents and the effect of sex on these associations remain poorly understood. PURPOSE: To examine the moderating effects of sex on the relationship between accelerometer-measured PA (volume, intensity) and EFs (inhibitory control, working memory) in 15-18 years old adolescents. METHODS: We analyzed baseline data from 417 participants (210 [50%] females, Mage = 16.5 ± 0.40 yrs, BMI = 23.2 ± 4.30 kg/m2) from the Burn 2 Learn cluster randomized controlled trial. Adolescents wore Actigraph GT9X Link accelerometers on their non-dominant wrist for 7 days, 24-h·d-1. Accelerometer metrics included average raw accelerations over 5 s epochs (AvgAcc), intensity gradient (IG; the slope of the log-log regression line of intensity over time; higher IG indicates higher intensity), and moderate-to-vigorous PA (MVPA) using Hildebrand youth cut-points. Inhibitory control was expressed as accuracy, reaction time (RT), and variability of the RT (standard deviation; SDRT) on incongruent trials of a modified Eriksen flanker task. Working memory (WM) was expressed as a d prime (d’) index on the 2-back task (z-scoretarget accuracy - z-scorefalse alarms). Multi-level random intercept models, accounting for clustering within schools, were used to test the moderating effects of sex on the PA-EF relationship, adjusting for age, BMI, and aerobic fitness. RESULTS: Sex moderated the relationship between IG and incongruent SDRT (b = 0.16, 95% CI: 0.04, 0.28, P = 0.01), and IG and the d’ index (b = -0.29, 95% CI: -0.47, -0.11, P = 0.002) even after controlling for AvgAcc. Girls with higher IG showed less variability in incongruent RT (b = -0.16, 95% CI: -0.25, -0.06; boys: Ps ≥ 0.68) and higher d’ index (b = 0.25, 95% CI: 0.11, 0.40; boys: Ps ≥ 0.21). In contrast, AvgAcc was not related to inhibitory control (Ps ≥ 0.06) or WM (Ps ≥ 0.21) after adjusting for IG. MVPA was not related to inhibitory control or WM (Ps ≥ 0.20). CONCLUSIONS: Our findings reveal a novel selective association between PA intensity and EFs among older adolescent girls. Girls with higher intensity profiles showed superior performance across two EF tasks.