A feature of student--teacher--scientist partnerships (STSPs) involves students working with scientists for the purpose of helping them learn more about how scientists work and think. Previous research on STSPs has generally focused on identifying the best practices of partnerships and on identifying challenges of these partnerships. The study reported here employed a cluster-randomized trial design to test the effectiveness of the PlantingScience STSP that combines high-quality curriculum, teacher preparation, and online mentoring by professional scientists. The results of the current study show that students who participated in the PlantingScience STSP showed significant improvements in science content knowledge and attitudes about scientists compared with students in the control group. The study sample was highly representative, demographically, to the U.S. population of high schools. These results add to the limited empirical evidence about the effectiveness of STSPs on student outcomes related to science achievement and attitudes.