The academic achievement of immigrant children has been a focus of social research for decades. Yet little attention has been paid to peer social capital and its importance as a school context factor for the academic success of immigrant youths. Using multilevel data from the National Longitudinal Study of Adolescent Health (Waves 1 and 3), this article draws upon social capital theory and assimilation theory to examine the effects of peer social capital on the academic achievement and attainment of immigrant and native youths. The effects of three measures of peer social capital are studied, controlled for many important variables, for example, sociodemographic background variables, school characteristics, and family social capital. Results indicate that only the average GPA (grade point average) of peers had a consistent and significant effect on children's achievement and attainment, whereas the density and the homogeneity of the peer network did not. Furthermore, all three measures of peer social capital have stronger effects for immigrant youths than for native youths.