University science, technology, engineering, and math (STEM) summer bridge programs provide incoming STEM university students additional course work and preparation before they begin their studies. These programs are designed to reduce attrition and increase the diversity of students pursuing STEM majors and STEM career paths. A meta-analysis of 16 STEM summer bridge programs was conducted. Results showed that program participation had a medium-sized effect on first-year overall grade point average (d = 0.34) and first-year university retention (Odds Ratio [OR] = 1.747). Although this meta-analytic research reflects a limited amount of available quantitative academic data on summer STEM bridge programs, this study nonetheless provides important quantitative inroads into much-needed research on programs’ objective effectiveness. These results articulate the importance of thoughtful experimental design and how further research might guide STEM bridge program development to increase the success and retention of matriculating STEM students.