Teachers' confidence and facility with strategies that position and support students who are English learners (ELs) as active participants in middle grades mathematics classrooms are key to facilitating ELs' mathematics learning. The Visual Access to Mathematics (VAM) project developed and studied teacher professional development (PD) focused on linguistically-responsive teaching to facilitate ELs' mathematical problem solving and discourse. This study examines whether VAM PD has a positive impact on teachers' self-efficacy in supporting ELs in mathematics and how components of the PD may have influenced teacher outcomes. Results from a field test involving a cluster randomized trial of 101 teacher participants from 47 schools showed that VAM PD had a positive impact on participants' self-efficacy related to teaching ELs in mathematics, based on pre/post self-efficacy survey responses. An analysis of participants' written reflections suggests that supported implementation of language strategies with ELs in their mathematics teaching contexts was a key PD component that contributed to teacher self-efficacy outcomes. Findings offer implications for mathematics teacher PD and for facilitating ELs' learning in mathematics.