Elementary school principals, as instructional leaders, are critical to improving mathematics proficiency. The research problem was that elementary school principals are inconsistently applying instructional leadership to support mathematics teachers’ efforts to improve students’ proficiency. The purpose of this qualitative case study was to understand how elementary school principals apply instructional leadership to support mathematics teachers in their efforts to improve student proficiency in mathematics. The conceptual framework was the instructional leadership model of Hallinger and Murphy, which defines three main dimensions of instructional leadership: (a) the school mission, (b) the instructional program, and (c) the school climate. The research question was how elementary school principals apply instructional leadership to support mathematics teachers to improve student proficiency in mathematics. The participants were 12 elementary school principals who were purposefully selected. Data were collected via semistructured interviews via Zoom using an interview protocol. Thematic analysis was used to analyze the data. The themes revealed that principals should set goals based on student achievement while fostering communication with all stakeholders. The themes are embedded in the job functions within instructional leadership’s three dimensions. Implications for positive social change include providing new information on how school principals can apply instructional leadership practices to better support mathematics teachers’ work with students. These efforts may help students to improve their proficiency in mathematics and graduate from school.