Persistence across undergraduate science, technology, engineering, and mathematics (STEM) programs is exceptionally low. Recent studies have shown that social support and sense of belonging are particularly important for students who are historically underrepresented in STEM, yet few interventions have directly targeted or investigated these factors. This qualitative study investigates low-income, high-achieving undergraduate STEM students' perceptions of their belonging in the context of a 2-year peer social support group intervention. Interview analysis of 11 participants demonstrates that these STEM students attribute their sense of belonging to feelings or displays of comfort, commonality, community, and concerted effort. The peer group facilitated increases in participants' social support and sense of belonging by allowing participants to build friendships, recognize shared experiences, connect to their campus, build confidence with peers, and feel supported in their non-academic and academic struggles. Although the program's main objective was to build participants' sense of belonging, the social support provided through the peer group also acted as a mechanism for increasing information-related social capital. We recommend the implementation of similar non-academic, supportive social spaces to increase the sense of belonging and overall persistence of low-income STEM students.