Background/Context: Newcomer immigrant students are entering schools in the United States in unprecedented numbers. As they enter new school contexts, they face a number of challenges in their adjustment. Previous literature suggested that relationships in school play a particularly crucial role in promoting socially competent behavior in the classroom and in fostering academic engagement and school performance. Purpose: The aim of this study was to examine the role of school-based relationships in engagement and achievement in a population of newcomer immigrant students. Research Design: The Longitudinal Immigrant Student Adaptation Study (LISA) used a mixed-methods approach, combining longitudinal, interdisciplinary, qualitative, and quantitative approaches to document adaptation patterns of 407 recently arrived immigrant youth from Central America, China, the Dominican Republic, Haiti, and Mexico over the course of 5 years. Based on data from the last year of the study, we examine how the role of relationships mediates newcomers' challenges with academic engagement and performance. We identify factors that account for patterns of academic engagement and achievement, including country of origin, gender, maternal education, English language proficiency, and school-based relationships. Findings: Multiple regression analyses suggest that supportive school-based relationships strongly contribute to both the academic engagement and the school performance of the participants. Qualitative interview data and case studies serve to elucidate the relational processes inside and outside school that influence different academic outcomes.