In this grounded theory case study, four interconnected, foundational cornerstones of culturally responsive mathematics teaching (CRMT), communication, knowledge, trust/relationships, and constant reflection/revision, were systematically unearthed to develop an initial working theory of CRMT that directly informs classroom practice. These cornerstones were found to interact in unique ways. Results have implications for teachers of mathematics who aim to become more culturally responsive, mathematics teacher educators and supervisors, and school administrators who seek to promote equity in mathematics.