According to Marzano (2017), classroom teachers are the single most important factor within a school’s locus of control that contributes to student learning. The purpose of this study was to explore the relationships between fifth-grade teachers’ support behaviors as measured by the Classroom Assessment Scoring System™ (CLASS™) and achievement of fifth-grade students in English language arts (ELA) and mathematics. The research design was a non-experimental, correlational exploration of archival data extracted from the Measures of Effective Teaching (MET) database. Mean CLASS™ composite, domain and dimension scores of 70 fifth-grade teachers were correlated to mean z scores of 70 fifth-grade students on summative assessments of ELA and mathematics. The results indicated that the mean CLASS™ score on the dimension of Regard for Student Perspectives within the Emotional Support domain was a significant predictor of ELA achievement. In addition, the dimensions of Behavior Management within the Classroom Organization domain and the dimension of Negative Interactions within the Emotional Support domain were significant predictors of mathematics achievement among fifth-grade students. Implications of the findings for teaching and learning and recommendations for further research are discussed.