Background: Studies in the United States show that school students from some ethnic backgrounds are susceptible to stereotype threat, that this undermines their academic performance, and that a series of virtually zero-cost self-affirmation writing exercises can reduce these adverse effects. In England, however, socioeconomic status (SES) is a much stronger predictor of academic success than is ethnic background. Aims: This study investigates whether self-affirmation writing exercises can help close the SES attainment gap in England by increasing the academic performance of low-SES (but not higher-SES) school students. Sample: Our sample consisted of students aged 11-14 in a secondary school in southern England (N = 562); of these, 128 were eligible for free school meals, a proxy for low SES. Methods: Students completed three short writing exercises throughout one academic year: those randomly assigned to an affirmed condition wrote about values that were important to them, and those assigned to a control condition wrote about a neutral topic. Results: On average, the low-SES students had lower academic performance and reported experiencing more stereotype threat than their higher-SES peers. The self-affirmation raised the academic performance of the low-SES students by 0.38 standard deviations but did not significantly affect the performance of the higher-SES students, thus reducing the SES performance gap by 62%. The self-affirmation also reduced the level of stress reported by the low-SES students. Conclusions: The benefits of this virtually zero-cost intervention compare favourably with those of other interventions targeting the SES academic attainment gap.