Women and racially and ethnically minoritized populations are underrepresented in science, technology, engineering, and mathematics (STEM). Out-of-school time programs like summer camps can provide positive science experiences that may increase self-efficacy and awareness of STEM opportunities. Such programs often use the same high-impact practices used in K-12 classrooms including relating concepts to real-world examples, engaging students as active participants in inquiry-driven projects, and facilitating learning in a cooperative context. They additionally provide opportunities for engaging in STEM without fear of failure, offer a community of mentors, and allow families to become more involved. We designed a summer camp for middle schoolers who identified as girls, low-income, and as a minoritized race or ethnicity. We describe the design of the camp as well as the results from a simple pre- and post-camp questionnaire that examined each camper's relationship to science, scientific self-efficacy, and interest in having a job in STEM. We found an increase in self-efficacy in camp participants, which is important because high scientific self-efficacy predicts student performance and persistence in STEM, especially for girls. We did not detect an increase in interest in pursuing a STEM job, likely because of already high values for this question on the pre-camp survey. We add to the growing body of work recognizing the potential of out-of-school time STEM programs to increase scientific self-efficacy for girls and racially minoritized students.