Students who come from low-income backgrounds tend to be underidentified and underserved in gifted education. Early interventions with learners of high potential from underserved groups, including exposure to challenging curriculum and summer opportunities, are important for nurturing these students' talents and preparing them for advanced learning opportunities in later school years. Project SPARK, based on the Young Scholars model, focuses on recognizing and responding to high-potential learners from underserved populations in the early grades. In this study, we examined the effects on mathematics achievement of participation in a summer program as part of Project SPARK in schools with substantial populations of students in poverty, as demonstrated by percentage of students eligible for free or reduced lunch. We examined summer program effects on achievement for the sample overall and specifically for students from low-income backgrounds. Students who participated in the summer program made moderately larger mathematics achievement gains than students who did not participate, with Cohen's d-type effect sizes of 0.92. Students who qualified for free or reduced lunch saw similar gains over the summer if they participated in the summer program, indicating that the summer program experience was supportive for students across a range of income backgrounds.