Using longitudinal data on a nationally representative U.S. cohort, this study investigated the relationship between socioeconomic status and students' reading growth between kindergarten and eighth grade. Piecewise latent growth modeling was used to describe nonlinear growth trajectories in reading during three developmental periods: kindergarten through first grade, first grade through third grade, and third grade through eighth grade. Results indicated that lower child SES was associated with faster rates of reading growth in the primary grades, but with slower rates of growth after third grade. Higher school concentration of students from low-SES backgrounds was also associated with slower rates of growth between third and eighth grade. Findings support increased attention from researchers and educators to reading achievement gaps after third grade.