Co-teaching may be a promising strategy to improve inclusive secondary mathematics education in rural schools. Professional development (PD) aids in special and general education teachers' co-teaching implementation, yet little empirical research examines how to effectively train and support co-teachers. In this study, we describe one U.S. state's PD model for secondary mathematics co-teaching, evaluate outcomes of the PD on co-teachers' behaviors and beliefs, and examine the impact of co-teaching on students' mathematics achievement. We examined data from observations, surveys, and students' pre/post assessments across nine classes within seven U.S. school districts, including three rural school districts, over 3 years. We report data from a total of 19 teachers and 281 students in sixth through ninth grades. The PD participants implemented effective co-teaching strategies and reported positive viewpoints toward co-teaching. Additionally, students with and without disabilities improved their mathematics scores. We present key findings for rural schools to guide future implementation and research.