Broadening participation in science, technology, engineering, and mathematics (STEM) is critical to the nation’s economic growth and national security. In K–12 and higher education, researchers and educators increasingly employ the concept of social capital to develop programs for improving STEM learning, motivation, and participation of young students. STEM social capital in education comprises STEM-oriented resources—whether instrumental, informational, or emotional—that students access through their social networks. Major theoretical perspectives, research evidence, and promising practices are associated with the concepts of social capital in STEM education. Students’ social capital in STEM education (derived from families, peers, teachers, and professional networks) demonstrably promotes their STEM educational outcomes and career paths. Inclusive STEM schools, mentoring, and after-school programs are some promising approaches that can enhance STEM social capital and outcomes of underrepresented students, particularly women, Blacks/Hispanics/Native Americans, youth with low socioeconomic status, and persons with disabilities.