Background: Despite knee extensor and flexor strength reportedly being associated with injury risk, including rupture of the anterior cruciate ligament in girls, there is limited evidence for the longitudinal changes in lower extremity strength. Purpose: To investigate the sex-specific relationship with longitudinal changes of knee extensor and flexor strength associated with maturation. Study design: Cohort study; Level of evidence, 2. Methods: Adolescent boys and girls (N = 257; 208 adolsecent girls) participating in high school basketball, volleyball, and soccer were assessed longitudinally in at least 2 different pubertal stages. Pubertal status (prepubertal, pubertal, and postpubertal) was determined with the modified Pubertal Maturation Observation Scale questionnaire. After a warm-up of 5 submaximal repetitions, participants were tested for concentric peak isokinetic strength for knee extension and flexion at 300 deg/s over 10 repetitions and normalized to body weight. Linear mixed models were used to test for the effect of pubertal stage, sex, and their interaction. Results: Significant interactions were identified that indicated different maturational trajectories for knee muscle strength for adolescent boys and girls, particularly between prepubertal and pubertal stages, in which boys demonstrated greater mass normalized knee extensor increases than girls (right, +12% vs +5%; left, +13% vs +7%; P <.001). For knee flexors, boys demonstrated increased strength, while girls demonstrated decreased relative knee flexor strength (right, +4% vs ���1%, P =.03; left, +3 vs ���3%, P =.009). Conclusion: The findings of this study support a differential effect of sex and maturation on important knee strength outcomes that may have implications for knee injury reduction, particularly in adolescent girls as they mature.