This paper examines the antecedents of three types of educational beliefs about mathematics among 151 teachers predominantly working in high poverty schools. Studies across various countries have found that teachers in high poverty schools are less likely to enact instructional approaches that align with mathematics reform standards set by national and international organizations. Researchers contend that for instruction to change, educational beliefs about mathematics and teaching must change. Regression analyses indicated that mathematics-teaching experience was associated with teachers' self-efficacy for teaching mathematics at the onset of professional development and the number of mathematics college courses teachers had taken moderated their change in self-efficacy beliefs through professional development. Findings also indicated that epistemic beliefs about mathematics, which became more availing through professional development, were the strongest predictor of their mathematical knowledge for teaching. Results may inform professional development programs in promoting adaptive educational beliefs among teachers in high poverty schools.