Social cognitive theory guided the design of a survey to investigate high school students' perceptions of factors affecting their career contemplations and beliefs regarding the influence of their participation in the international Science Olympiad on their subject interests and twenty-first century skills. In addition, gender differences in students' choice of competition category were studied. Mixed methods analysis of survey returns from 172 Olympiad participants from 31 countries showed that students' career aspirations were affected most by their teachers, personal interests, and parents, respectively. Students also indicated that they believed that their participation in the Olympiad reinforced their plan to choose a science, technology, engineering, and mathematics (STEM) major at college and assisted them in developing and improving their twenty-first century skills. Furthermore, female students' responses indicated that their project choices were less likely to be in the engineering category and more likely to be in the environment or energy categories. Findings are discussed in the light of increasing the awareness of the role and importance of Science Olympiads in STEM career choice and finding ways to attract more female students into engineering careers.