Statistical and data literacy have emerged as key competencies in the 21st-century. Science, technology, engineering, and mathematics (STEM) curricula have been adapted in various countries to place increased emphasis on proficient data use and statistical comprehension. Consequently, there is a need for an adequate education and professional training of STEM teachers. This systematic review analyzes and synthesizes research on teachers' statistical and data literacy in K-12 STEM education. A total of 42 articles were analyzed, with a specific focus on teacher variables, teacher education, and their impact on classroom practice. The findings emphasize the recent emergence of research in this domain, particularly focusing on statistical literacy, with mathematics being the dominant STEM domain explored. Most studies examined cognitive variables of both pre-service and in-service teachers, revealing relevant gaps in knowledge and skills related to statistical and data literacy. Limited attention was given to teachers' affective variables. Research on the complex relationships between teacher variables, their teaching practices, and their students' learning is scarce, necessitating further investigation. Several pedagogical approaches to prepare STEM teachers for promoting statistical and data literacy in their classrooms were identified. Positive impacts on teachers' cognitive and affective variables were observed, but longitudinal effects and transfer to classroom practice require further inquiry. As curricula worldwide increasingly emphasize statistical and data literacy, this review underscores the necessity of enhancing teachers' competencies, advocates for tailored pedagogical approaches, and emphasizes the need for further research to shape effective instruction in K-12 STEM education.