We examined the associations of autonomous motivation, controlled motivation, and physical activity (PA) planning with PA participation over six years across the adolescent-to-adult transition. Participants from the NEXT Generation Health Study, a nationally representative cohort study of U.S. 10th graders (N=2785), completed surveys yearly from 2010 to 2016 (four years post-high school). This study used data from Waves 2 (W2) through 7 (W7). Data were analyzed using growth models accounting for the complex survey design and controlling for sex, race/ethnicity, and body mass index. A piecewise growth model with two pieces (Piece 1: W2-W4; Piece 2: W4-W7) indicated that PA declined during late adolescence (W2-W4) (b=-0.31, β=-0.22, p<.001), but did not decline after the transition into early adulthood (W4-W7) (b=-0.08, β=-0.04, p=.052). Autonomous motivation was positively associated with PA at all waves (b=0.23-0.33, β=1.90-4.37, p<.001). Controlled motivation was only positively associated with PA at W3 (12th grade) (b=0.13, β=1.54, p=.011). PA planning varied significantly between individuals and significantly predicted PA (b=0.44, β=0.21, p<.001). Although PA decreased significantly during late adolescence, PA did not decrease significantly after transitioning into early adulthood (one to four years post-high school). Elevated autonomous motivation and PA planning were consistently and significantly associated with higher PA, suggesting that these may be useful intervention targets during this adolescent-to-adult transition.