Informal science, technology, engineering, and math (STEM) programs are important tools for broadening participation in STEM careers. The St. Jude STEM Club (SJSC) is a 10-week afterschool STEM club focused on real-world problems in pediatric cancer research and designed for students in the fifth grade. The SJSC is conducted in partnership with the Shelby County Schools (SCS), an urban school district that encompasses Memphis, TN and serves a disproportionate number of students from underrepresented backgrounds in science. In this report, we provide details on the club logistics, curriculum, pilot data and outcomes related to club impact on student attitudes towards science, and challenges and limitations of the program. Participants in the program reported significantly higher rates of STEM engagement, STEM identity, critical thinking, perseverance, and relationships with peers and adults compared to national normative data. This program description is intended to serve as a resource for other institutions wanting to use a similar strategy to broaden participation in STEM careers.