The past decade has brought increased focus on STEM learning (Bell, Lewenstein, Shouse, & Feder, 2009; NGSS Lead States, 2013; U.S. Department of Education, 2015). The growth of STEM-related industries and the power associated with STEM fields make access to STEM careers an equity issue (Buechley, 2016). Despite gains in educational achievement, women and individuals from nondominant cultures remain underrepresented in STEM majors and careers (National Science Foundation, 2017). Afterschool programs offer a promising context for engaging diverse students: African American and Latinx children attend afterschool programs at rates twice that of White students (Afterschool Alliance, 2015). STEM programs at youth-centered sites capitalize on the resources of spaces children find welcoming and accessible. The natural curricular flexibility of afterschool programs enables immersive exploration and experimentation in STEM as well as authentic opportunities for building skills and developing relationships helpful to STEM careers (Afterschool Alliance, 2015; Krishnamurthi, Ballard, & Noam, 2014). Afterschool science programs naturally blur disciplinary boundaries and incorporate diverse ways of knowing (Calabrese Barton, Birmingham, Sato, Tan, & Calabrese Barton, 2013). These factors can be leveraged to broaden young people's definition of science and to foster "productive hybrid STEM identity work for underrepresented youth" (Calabrese Barton, Tan, & Greenberg, 2017, p. 21). Science education in youth-centered sites can value the cultures of underrepresented students while encouraging them to explore new science-related interests and identities (Calabrese Barton & Tan, 2010). Despite widespread acceptance of the benefits of afterschool STEM, more research is needed on how program factors affect student engagement and learning (Laursen, Thiry, Archie, & Crane, 2013). Coburn and Penuel (2016) call for more studies on program processes, collaboration strategies, and productive responses to challenges. The STEMinist Program, created by the authors, was a research-practice collaboration between university researchers and an afterschool program for female students in grades 4 to 6. This article describes how the program's ongoing design transformations increased girls' understanding of and interest in STEM. Design-based framing (Barab & Squire, 2009) enabled ongoing adjustments to the program while also identifying best practices for afterschool STEM learning. To understand the program's progression and outcomes, the authors examined the features of the learning environment and the relationships among design components by analyzing qualitative data collected before, during, and after program implementation. Participants' perceptions of science and scientists helped them understand the impact of the program and ways to improve it.