The relationship between a student's sense of belongingness and their academic achievement in high school has been well established in the research literature. However, few studies have explored this relationship among immigrant students. This study hypothesizes that belongingness and achievement for immigrant students is specifically mediated by peer and teacher influences. Therefore, engagement with academically inclined peers, and teachers who share an ethos of commitment toward their students, may result in both belongingness and achievement for immigrant students. In addition, a student's language status, which may help to develop social relationships, may also play an important role. Therefore, an investigation on the effect of peers, teachers and the English-Language status of immigrant students may clarify the link between belongingness and achievement. This dissertation uses the High School Longitudinal Study of 2009 (HSLS:09) to test a model that accounts for premigration measures including language status, while including the effect of peers, and teachers, on sense of belongingness and academic achievement. The analysis for this study is organized into two parts. In Study 1, belongingness is treated as an outcome and the effects of peers, teachers and English-language status are regressed on sense of belongingness. In Study 2, belongingness is treated as a predictor variable and the outcome is a student’s high school GPA. In the first study, the results showed statistically significant results for the effect of peers and teachers on sense of school belongingness. In the second study, the effect of peers, teachers and sense of school belongingness were statistically significant predictors of academic achievement. The limitations of this study and the implications for future research are also discussed.