This paper evaluates the academic impact of a growth-mindset intervention on students starting the secondary level in public schools in urban Peru. ¡Expande tu Mente! is a 90-minute school session aimed at instilling the notion that a person's own intelligence is malleable. Students in schools randomly assigned to treatment showed a small improvement in math test scores and educational expectations, with a large and sustained impact in test scores among students outside the capital city. At a cost of $0.20 per pupil, ¡Expande tu Mente! was highly cost-effective. The results show the potential that brief growth-mindset interventions have for developing countries. [This paper is a product of the Poverty and Equity Global Practice.]