There is growing enthusiasm for STEAM education in preparing students for an increasingly complex world. However, implementing STEAM in the classroom can be challenging for educators, as it may require collaboration across disciplines, increased workload, and understanding the nature of STEAM integration. This paper details a mixed-methods evaluation of a year-long STEAM teacher training program, in which a STEM teacher and an arts teacher collaborated to design and implement integrated STEAM lessons at each of the nine participating schools (n = 17). The training program consisted of a 5-week summer professional development experience, followed by ongoing financial, material, and pedagogical support during the school year, made possible by the partnership of the schools, a university, and community organizations. Findings from surveys, focus groups, and written reflections suggest that, despite certain challenges, aspects of the training program supported teacher implementation of STEAM. Participation in the program impacted teachers' collaboration, pedagogy, self-efficacy, and arts integration practices. The findings offer insight into the forms of support that teachers deem important in STEAM teacher training programs and the benefits of such a program for teachers' professional development.