Legal professionals have access to many different sources of knowledge, including user-generated Wikipedia articles that summarize previous judicial decisions (i.e., precedents). Although these Wikipedia articles are easily accessible, they have unknown provenance and reliability, and therefore using them in professional settings is problematic. Nevertheless, Wikipedia articles influence legal judgments, as we show using a first-of-its-kind randomized control trial on judicial decision making. We find that the presence of a Wikipedia article about Irish Supreme Court decisions makes it meaningfully more likely that the corresponding case will be cited as a precedent by judges in subsequent decisions. The language used in the Wikipedia article also influences the language used in judgments. These effects are only present for citations by the High Court and not for the higher levels of the judiciary (Court of Appeal and Supreme Court). The High Court faces larger caseloads, so this may indicate that settings with greater time pressures encourage greater reliance on Wikipedia. Our results add to the growing recognition that Wikipedia and other frequently accessed sources of user-generated content have profound effects on important social outcomes and that these effects extend farther than previously seen—into high-stakes settings where norms are supposed to restrict their use. History: Rajiv Kohli, Senior Editor; Gordon Burtch, Associate Editor. Funding: The authors thank the Massachusetts Institute of Technology and Maynooth University for research funding. Supplemental Material: The online appendix is available at https://doi.org/10.1287/isre.2023.0034 .