A longitudinal science intervention with students from ethnic-racial underrepresented groups in an urban area examined the roles of intervention participation and STEM relationships in implicit and explicit science identity and attitudes and social belonging. Across a four-week geoscience program, Black, Latinx, and Native American/Alaskan Native (87.5%) students (N = 97; M[subscript age] = 15.27; female = 44%) from low socio-economic backgrounds engaged in hands-on activities, field trips, group projects, and listened to diverse speakers. During the intervention, students had the opportunity to form relationships with teachers and near-peer mentors (undergraduate STEM students). Participants exhibited increases in positive explicit and implicit science attitudes, identity, and social belonging. Also, psychosocial support from teachers and near-peer mentors developed over time, but near-peer mentorship uniquely explained changes in science identity and social belonging. Positive changes in implicit and explicit attitudes and explicit science identity were further qualified by past academic performance--only low, compared to high, achieving students benefited the most from the intervention. The present intervention provides evidence that immersing ethnic-racial minority high school students in an engaging science program with supportive STEM relationships promotes science-based cognitions that have implications for persistence in STEM.