Information sharing is a ubiquitous social behavior. What causes people to share? Mentalizing, or considering the mental states of other people, has been theorized to play a central role in information sharing, with higher activity in the brain’s mentalizing system associated with increased likelihood to share information. In line with this theory, we present novel evidence that mentalizing causally increases information sharing. In three pre-registered studies (n = 400, 840, and 3500 participants), participants who were instructed to consider the mental states of potential information receivers indicated higher likelihood to share health news compared to a control condition where they were asked to reflect on the content of the article. Certain kinds of mentalizing were particularly effective; in particular, considering receivers’ emotional and positive mental states, led to the greatest increase in likelihood to share. The relationship between mentalizing and sharing was mediated by feelings of closeness with potential receivers. Mentalizing increased feelings of connectedness to potential receivers, and in turn, increased likelihood of information sharing. Considering receivers’ emotional, positive, and inward-focused mental states was most effective at driving participants to feel closer with potential receivers and increase sharing. Data provide evidence for a causal relationship between mentalizing and information sharing and provide insight about the mechanism linking mentalizing and sharing. Taken together, these results advance theories of information sharing and shed light on previously observed brain-behavior relationships.