Background: Students living in rural areas of the United States exhibit lower levels of educational attainment than their suburban counterparts. Innovative interventions are needed to close this educational achievement gap. Aims: We investigated whether an online growth mindset intervention could be leveraged to promote academic outcomes. Sample: We tested the mindset intervention in a sample of 222 10th-grade adolescent girls (M age = 15.2; 38% White, 25% Black, 29% Hispanic) from four rural, low-income high schools in the Southeastern United States. Methods: We conducted a randomized controlled trial to test the efficacy of the growth mindset intervention, relative to a sexual health programme. We used random sampling and allocation procedures to assign girls to either the mindset intervention (n = 115) or an attention-matched control programme (n = 107). We assessed participants at pre-test, immediate post-test, and 4-month follow-up. Results: Relative to the control condition, students assigned to the mindset intervention reported stronger growth mindsets at immediate post-test and 4-month follow-up. Although the intervention did not have a total effect on academic attitudes or grades, it indirectly increased motivation to learn, learning efficacy and grades via the shifts in growth mindsets. Conclusions: Results indicate that this intervention is a promising method to encourage growth mindsets in rural adolescent girls.