This meta-analysis examined the effectiveness of teacher professional development (PD) training on improving in-service STEM teachers' self-efficacy in the K-12 teaching context. Twenty-one articles yielded 48 effect size estimates, incorporating 1412 teachers in the final analyses. Results indicated that the overall effect size of PD training on K-12 STEM teachers' self-efficacy was strong at g = 0.64 (p < 0.01). This review contributes to the STEM teacher self-efficacy literature and confirms the significant impact of PD on improving STEM teachers' self-efficacy. Results regarding the impact of substantive PD characteristics provide policymakers and practitioners seeking to improve STEM teacher self-efficacy with information for improving PD. Recommendations for researchers are also discussed in the paper.