Background: Although spatial ability and mathematics performance are highly correlated, there is scant research on the extent to which spatial ability training can improve mathematics performance. Aims: This study evaluated the efficacy of a visuospatial intervention programme within classrooms to determine the effect on students' (1) spatial reasoning and (2) mathematics performance as a result of the intervention. Sample: The study involved grade six students (ages 10-12) in eight classes. There were five intervention classes (n = 120) and three non-intervention control classes (n = 66). Methods: A specifically designed 10-week spatial reasoning programme was developed collaboratively with the participating teachers, with the intervention replacing the standard mathematics curriculum. The five classroom teachers in the intervention programme presented 20 hr of activities aimed at enhancing students' spatial visualization, mental rotation, and spatial orientation skills. Results: The spatial reasoning programme led to improvements in both spatial ability and mathematics performance relative to the control group who received standard mathematics instruction. Conclusions: Our study is the first to show that a classroom-based spatial reasoning intervention improves elementary school students' mathematics performance.