The usefulness of college admissions test scores and high school GPA for predicting college success, particularly in STEM (science, technology, engineering, and mathematics), has been widely debated. As colleges across the country face challenges in STEM student retention and achievement, understanding the factors that contribute to college success is crucial. Standardized tests, such as the ACT®, have a long history of being used as indicators of college readiness. In the present study, the author finds evidence that high school GPA (HSGPA) and ACT® STEM scores are both independently significant predictors of first year GPA (FYGPA) for STEM majors, with HSGPA generally having a stronger individual effect. The study also found that the interaction between HSGPA and ACT STEM scores enhances predictive accuracy which demonstrates a complex and synergistic relationship. The study emphasizes the importance of sustained high performance in high school as well as equitable access to resources for students from diverse backgrounds. The study also highlights the complementary role of HSGPA and ACT STEM scores in predicting FYGPA for STEM majors. This study also reaffirms the importance of rigorous high school preparation.