This study examines the impact of interdisciplinary teacher collaboration on English-medium instruction (EMI) teachers' professional development in higher education. Five content teachers from a variety of academic backgrounds jointly designed, developed and implemented an interdisciplinary EMI course at a private university in Taiwan. Based on classroom observations, in-depth interviews and document analysis, the findings show that content teachers' attitudes towards EMI teaching became more positive as a consequence of participating in teacher collaboration. The findings also stress the value of teacher collaboration in supporting content teachers' language teaching strategies and instructional language use in EMI. Implications and directions for future research are also provided.