Abstract: Adolescence represents a developmental period of waning academic motivation, particularly in STEM domains. To combat this, better understanding the factors that might foster STEM motivation and interest is of importance. Social factors like social competencies and feelings of belonging become increasingly important in adolescence. The current study investigated structural relations between social competencies, feelings of belonging to an informal STEM learning program, math and science efficacy and interest in a sample of 268 adolescents (Mage = 15.25; 66.8% girls; 42.5% White British or European American, 25.7% South Asian British or Asian American, 15.7% Afro-Caribbean Black British or African American 5.6% Bi-racial, and 3.0% other). Adolescents were recruited from six different informal learning sites (e.g., science museums, zoos, or aquariums) in the United States (n = 147) and the United Kingdom (n = 121). The results revealed positive relations between social competencies and belonging, and between belonging and math and science efficacy and interest. Further, the results also indicated a positive indirect effect of social competencies on efficacy and interest, via belonging. These findings have implications for guiding informal STEM programming in ways that can enhance STEM motivation and interest.