Background and Context The increasingly widespread use of social media to expand one's social connections is a relatively new but important phenomenon that has implications for teaching, learning, and teachers’ professional knowledge and development in the 21st century. Educational research in this area is expanding, but further investigation is necessary to better determine how to best support teachers in their professional development, collaboration, and classroom teaching. Prior literature reviews have focused extensively on higher education settings or particular platforms or platform types (e.g., Facebook, microblogging). This article provides needed insights into K–12 settings and encompasses work from a variety of social media types. We describe a systematic review of more than a decade of educational research from various countries to present the state of the field in K–12 teachers’ use of social media for teaching and professional learning across various platforms. Research Questions To define social media's potentially beneficial roles in teaching and learning, we must first take an in-depth look at teachers’ current social media practices. Toward this end, we approached our review with the following research question: How are social media perceived and used by K–12 teachers for their teaching or professional learning, and with what impacts on teachers’ practices? Research Design Guided by Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) established standards for rigor and quality in systematic literature reviews, this article reviews empirical research to examine how social media are perceived and used by K–12 teachers with what impacts on teachers’ practices. Findings We find that social media features offer several benefits for helping teachers fulfill their goals for classroom teaching, including enhancing student engagement, community connections, and teacher–student interactions, but these affordances come with challenges that must be navigated. The literature also suggests that social media features provide benefits for teachers’ professional learning within both formal professional development programs and informal learning networks. Conclusions Implications of this literature review for future research and the design of educational practices are discussed in the final section. Among our conclusions are calls for more data triangulation between teachers’ and students’ learning and experiences on social media, more attention to teachers’ observational behaviors on social media, and further exploration of how social media facilitates interplay between teachers’ formal and informal learning.