In recent years, there has been increasing emphasis on integrated STEM education, reflecting the fact that the four STEM disciplines (i.e., science, technology, engineering, and mathematics) are often integrated in real-world applications. However, most K-12 teachers are trained within their own subject discipline and may not be capable of implementing an integrated approach to STEM education. There is therefore a need to develop teacher professional development (TPD) programs that can provide high-quality learning opportunities and support for teachers. The overarching goal of this research synthesis is to develop a set of design principles for effective TPD for integrated STEM education. To this end, this paper reviews 48 empirical studies and identifies the elements of effective TPD and potential challenges to implementing integrated STEM education. Content knowledge, pedagogical content knowledge, and sample STEM instructional materials are the three most frequently reported elements of effective TPD programs. However, even with TPD, teachers encounter various obstacles to the implementation of integrated STEM education, including pedagogical challenges (e.g., teachers' limited STEM knowledge) and structural challenges (e.g., teachers' lack of preparation time and resources). Based on the findings of this review, a set of design principles (e.g., allocate TPD time for teachers' microteaching) is proposed. This review contributes to the design and implementation of TPD programs by leveraging studies of the effective elements of TPD and addressing the potential challenges to integrated STEM education.