To optimize teacher support, administrators need subject-specific knowledge, skills, and beliefs to lead, not just manage, instructional change. Professional development (PD) is needed with the many roles administrators are already called to fill. In this study, 38 elementary administrators participated in PD in mathematics instructional leadership. Findings from the Administrator Self-Efficacy Survey for Mathematics, derived from the Principal Self-Efficacy Survey (Smith & Guarino, 2005), show that subject-specific, district-led PD designed around sources of self-efficacy (Bandura, 1977, 1982, 1986) had a significant impact on increasing administrators' mathematics instructional leadership self-efficacy. Specifically, administrators increased their beliefs to influence effective mathematics practice, apply district PD to instructional leadership practices, and justify change during reform. These results show promise for future mathematics-specific instructional leadership development. Results from this study can inform PD design for districts and leaders aiming to promote and support school administrators as instructional leaders and advocates for evidence-based practice.