Nearly one half of students planning to earn a STEM degree either change majors or leave college without a degree. The completion percentage is much lower for underrepresented minorities, of whom less than one third complete their STEM degree within six years. At the University of Wisconsin-Whitewater, the STEM Boot Camp program was designed to improve the retention of underrepresented minorities in STEM majors, primarily by preparing incoming freshman for introductory gateway STEM courses in their freshman and sophomore years. The program (a two-week on-campus summer program with additional benchmarks through the subsequent fall semester) incorporated peer and faculty mentorship, mock lectures, faculty office hours and student projects. Students participated in workshops on learning strategies and growth mindset, discussions on expectations and potential challenges, and integration of students into academic culture. Program participants showed significantly higher pass rates in two of six focal STEM courses and significantly increased retention rates to year two compared to non-participant underrepresented minority students. Participants also passed 100% of course retakes in focal STEM courses. Participants performed similarly or better than non-underrepresented minority students, supporting the effectiveness of the program in reducing or eliminating the achievement gap through introductory STEM courses.