Motivation to learn science plays an important role in the persistence, achievement, and career aspirations of students studying science, technology, engineering and math (STEM). Several studies have focused on how supplemental summer programmes build science motivation in undergraduate STEM students, but few studies have focused on how such programmes build motivation in high school students. This study assessed how the Emerging Scholars Environmental Health Sciences Academy, a supplemental summer science programme for public high school students in Louisiana, Alabama, and Florida in the southern United States, influenced participants' science motivation, as measured by the Science Motivation Questionnaire II (SMQII). The SMQII assesses five motivation factors for learning science: intrinsic motivation, self-determination, self-efficacy, career motivation, and grade motivation. Participants (n = 45) completed the SMQII before and after completing the academy, which allowed students to learn, apply, and experience environmental health science. Results indicated a statistically significant increase in motivation to learn science among participants, particularly for self-determination and career motivation. Inquiry-based learning, individual mentoring, and exposure to environmental health careers may explain why the academy was effective in positively influencing participants' self-determination and career motivation. Finally, practical lessons learned for implementing a supplemental science programme for high school students are provided.