An extensive body of literature explores the impacts of teaching professional development (TPD) on graduate students in the STEM disciplines. However, these TPD interventions are often extremely diverse and highly contextual. The purpose of this meta-analysis was to synthesize what is known about the impacts of TPD on select instructional outcomes of STEM graduate student teaching assistants. A final sample of N = 25 articles (published between 1990 and 2020) was analyzed to determine the effect of TPD interventions on instructional outcomes related to graduate students. These outcomes were categorized utilizing a previously published evaluation taxonomy and included the categories of graduate student teaching self-efficacy, graduate student knowledge and beliefs about instruction, graduate student instructional practices, and undergraduate student perceptions of graduate student instructional practices. Results demonstrated the positive impact of TPD interventions on STEM graduate student teaching self-efficacy (ES = 0.863, 95% CI [0.400-1.326], p = .006) with inconclusive results for other TPD outcome variables. The findings suggest that TPD interventions for STEM graduate students are promising for impacting participants' self-efficacy but more evidence is needed to understand the broader impacts of TPD interventions on other relevant instructional outcomes.