The aim of this study was to examine the role of motivational beliefs in mediating the relationship among perceived teacher support, learning strategy use, and student achievement. The author analyzed the Programme for International Student Assessment mathematics scores and questionnaire responses of 4,855 15-year-old students in Turkey via multilevel analysis. Results indicated that perceived teacher support was positively related to learning strategy use in mathematics and that this relation was mediated through math self-efficacy, anxiety, intrinsic value, and instrumental value. Math self-efficacy and anxiety were, in turn, correlated with Programme for International Student Assessment mathematics achievement. In addition, the author found between-school SES differences to be strong predictors of math self-efficacy, anxiety, and achievement. Findings and directions for future research are discussed. (Contains 4 tables and 2 figures.)