High school is a critical period for science identity development. Teacher influences are important but under-researched. Teachers' growth-mindset practices may help students meet the growing challenges to their ability, achievement, and identity in science and have long-term influences, especially for underrepresented students. We explored U.S. high school students' perceptions of science teachers' growth-mindset practices at ninth grade and the effect on the mean development trajectory of their science identity over six years. Three waves of data from a nationally representative sample of 15,648 students were analysed through longitudinal multilevel models. Data from underrepresented ethnic minorities (n = 4,485), students with low socioeconomic status (low-SES) (n = 2,219), and female students (n = 7,823) were analysed separately from the overall sample. Perceived teachers' growth-mindset practices significantly predicted initial science identity at ninth grade for the overall sample, ethnic minorities, and female students, as well as science identity development over time for all four samples, before controlling student competence beliefs, gender, race, and SES. The long-term effect was more salient for the ethnic minority and low-SES students. We extended prior work by examining the effect of perceived teachers' mindset practices in the science discipline, particularly the long-term effect, while most research on STEM motivation and identity from a growth mindset perspective focused on student mindset, the math discipline, and the short-term effect of mindset. Results suggest continued teacher training on growth mindset for helping students develop science identity, especially for ethnic minority and low-SES students.