STEM (science, technology, engineering, mathematics) hackathons have been conducted to offer opportunities for diverse participants to work collaboratively and intensely on tasks within a short period. However, most literature about hackathons does not offer theoretical discussions on the affordances of such learning contexts for school students. This paper reports on an inaugural event of a STEM capital-building program designed for students with relatively less access to STEM capital than their peers. Using epistemic infrastructure (EI) as the theoretical framework, we unpack the affordances of a STEM hackathon in terms of its cultural space, embodiments of artifacts, and practices (three tenets of EI) in building STEM capital. A total of 115 Grade 9 Singapore students from seven public schools participated in a purposeful 3-year STEM capital-building program. The data collected include video recordings of the activities, interviews with students and teachers, and photographs. The qualitative data were analyzed using prescriptive codes identified from the synthesis of the literature about epistemic infrastructure. The findings illuminated the three tenets of EI, and an additional tenet on social agency in EI was identified and discussed. The study contributes to the theorization of STEM curriculum and has implications for the design and implementation of informal STEM curriculum such as hackathons for STEM capital building.