Background: The current study aimed to investigate the associations between cognitive abilities, grit, and academic achievement in high school STEM studies. The study utilized a natural sample of excelling STEM students (N = 814) who underwent a selection process to join elite undergraduate STEM programs. Through this selection process, the students reported their high school grades in Math, Physics, and Computer science, as well as their Psychometric Entrance Test (PET) scores, which are equivalent to the SAT. They also completed a Grit questionnaire and took various cognitive ability tests. Results: Our study highlights the significant role of grit in STEM achievements, particularly for students with moderate cognitive abilities. We found that while cognitive abilities were strong predictors of academic STEM success, grit weakly contributed to predicting achievements in STEM studies beyond cognitive abilities alone. The study's main finding revealed that grit moderates the association between cognitive abilities and academic STEM grades, with higher levels of grit diminishing the impact of cognitive abilities on STEM success. Specifically, students with high grit could leverage their determination and passion to bridge cognitive gaps, effectively reducing performance disparities. Conclusions: The findings underscored the multifaceted nature of cognitive skills and non-cognitive factors like grit in contributing to STEM success. The study highlights the importance of fostering grit, particularly among students with moderate cognitive abilities. Higher grit can especially benefit these students in achieving higher STEM grades. According to Social Cognitive Career Theory (SCCT), higher achievements could promote one's self-efficacy and encourage students to pursue STEM careers after high school. This underscores the importance of integrating both cognitive and non-cognitive factors in educational and career guidance programs.