Individual differences in grit and growth mindset predict effort and achievement in the face of challenges, but little is known about how the two traits influence each other during adolescence. In the current investigation, we analyzed data on grit and growth mindset collected from 1667 adolescents and their teachers on four occasions over 2 academic years. In autoregressive cross-lagged models, grit predicted rank-order increases in growth mindset and growth mindset predicted rank-order increases in grit. These findings suggest that during adolescence, grit and growth mindset are distinct but mutually reinforcing.