The demands of learning in the twenty-first century require being skilled in the use and comprehension of multiple documents. Some individual factors such as the metacognitive skill of theory of mind (ToM) are related to this ability. This study investigated the relationship between university students' ability to comprehend multiple documents, measured through an argumentative essay task, and their ToM. The participants were 84 undergraduates. We assessed their ToM (using the strange stories task) and their mental state talk (using a think-aloud protocol during the reading of multiple documents--MST[subscript R]--and through the content of their subsequent written essays--MST[subscript W]). The relation between students' ToM and their MST[subscript W] was mediated through MST[subscript R]. In addition, students' ToM was significantly associated with the quality of the essays, through MST[subscript R], and the relation between MST[subscript R] and the argumentative quality was mediated through MST[subscript W]. MST[subscript W] was the only significant predictor of the quality of the essays. The involvement of ToM in multiple-document comprehension suggests, from an educational perspective, the possibility to support multiple-document comprehension by scaffolding mental state talk.