Although students who self-assess effectively often learn better, creating effective, low-cost interventions to help them do so is a critical challenge. This study examined the effects of a self-assessment diary intervention on 74 Form 1 (M[subscript age]=12.2 years) students' academic achievement, self-regulation, and motivation. After each homework assignment, students in the experimental group (n=37) completed a standardised self-assessment diary, while students in the control group (n=37) did no additional work. "Difference-in-differences" analyses showed that self-assessment diaries significantly enhanced students' academic achievement, self-efficacy, and intrinsic value. Students with lower past achievement benefited more than other students from the intervention. The intervention had no significant impact on effort regulation and self-reflection. Furthermore, effort-regulation, self-reflection, self-efficacy, and intrinsic value all did not mediate the link between self-assessment diaries and academic achievement. The findings can inform researchers and educators aiming to help students self-assess effectively to improve their learning.