Rubrics, which are designed to inform learners about assessment criteria and quality levels concerning a task assignment, are a widely used means to support learners in self-assessing their task performance. Usually, rubrics enhance task performance. Surprisingly, however, relatively little is known about the underlying mechanisms via which rubrics exert their beneficial effects. Although it is frequently suggested that the effects of rubrics on task performance are due to the fact that they support learners in accurately self-assessing their level of performance, which, in turn, paves the way for effective regulation, the empirical evidence that supports this notion is relatively scarce. Tightly controlled experimental studies, which allow isolating the effects of rubrics on the accuracy of self-assessments, are scarce. The present study was designed to close this gap. Specifically, in an experiment with N = 93 high school students we tested whether providing learners with a rubric in self-assessing the quality of self-written scientific abstracts would enhance judgment accuracy. As main results, we found that the rubric group showed higher absolute accuracy and lower bias. Furthermore, the rubric reduced cognitive load in self-assessing. These findings support the notion that one mechanism via which rubrics unfold their efficacy is by enhancing judgment accuracy.