Retention of students to science, technology, engineering, and mathematics (STEM) major has been studied for four cohorts participating in a summer bridge program supported by the National Science Foundation. Students participated in a 6-week program prior to their first term of enrollment at a research-intensive land grant university. Comparisons to baseline incoming classes from 2009–2011 show gains in retention to major out to the third year for underrepresented minorities (African American, Hispanic, Native American), first-generation college students, and females. Gains for underrepresented minorities and females are statistically significant, whereas those for first-generation students are not. These gains are associated with participation in a 6-week summer bridge program and the resulting improvement in preparation for college coursework, sense of belonging as measured by qualitative surveys, and use of academic support structures.