Teacher knowledge needed for teaching is widely studied to characterize its key categories. We report findings from a study on teachers' knowledge for mathematics in the Montessori schools. In Montessori accredited schools, teachers learn to teach mathematics in ways different from the teachers themselves experienced in non-Montessori schools. We ask: What knowledge do teachers learn? and how do they continue to refine this knowledge in teaching in classrooms? We draw from a teacher knowledge framework based on cross-national studies to interpret mixed data from a case study. We aim to inform research on teacher characteristics needed for consistent implementation of instructional reform. Major findings from this study are that for K-6 Montessori teachers to thrive in teaching mathematics in Montessori classrooms, they need teacher knowledge on Montessori materials, on lesson and the presentation of content according to Montessori's philosophy and pedagogy; as well as on the process of independently understanding concepts to be presented. The findings contribute to further theorizing on teacher knowledge which has implications is designed to teacher training opportunities in three subcategories; namely teaching, learning, and professional competence knowledge.