Despite intense interest in how social capital can facilitate informal learning, few attempts have been made to synthesize the interface between social capital and informal learning. Social capital is an important factor in the application of informal learning, especially in postsecondary science, technology, engineering, and mathematics (STEM) education. This paper first presents a review of the literature that has explored the relationship between social capital and informal learning in STEM education. We then present two case studies illustrating how social capital plays a pivotal role in enhancing informal learning outcomes in postsecondary STEM education. These cases offer insights on how to promote optimal informal learning in STEM education. Implications for understanding the importance of social capital in informal learning are discussed.