Studies show math achievement to be the best predictor of entering post-secondary education. However, less is known about the predictors of math achievement, particularly among immigrant youth. This study examined English proficiency and peer interethnic relations as predictors of mathematics achievement among Latino and Asian high school students, postulating an interaction between the predictors and mediation by academic motivation. A multilevel moderated-mediation model was used to analyze data from a national sample of 2113 non-native English speaking Latino and Asian students attending high school in the U.S. We found that higher academic motivation mediated the relationship between English proficiency during their sophomore year and gains in senior math achievement scores for both Asian and Latino students. For Latino students however, this indirect path was only significant for students whose perceptions of positive peer interethnic relations at school were average or above average. (Contains 2 figures and 2 tables.)