Knowing the geographic distribution of any species is an important first step toward understanding its ecology. Long-tailed weasels (Mustela frenata) are secretive and difficult to detect, so little is known about their distribution in Illinois. To fill this gap in the knowledge, we solicited information from museum curators about the sites where long-tailed weasel specimens were collected in Illinois and surveyed biologists and recreational trappers to document locations of where they had seen or captured long-tailed weasels. We present an updated map of long-tailed weasel distribution, documenting presence in at least 60 of the 102 counties in Illinois. Long-tailed weasels were seen or captured in a variety of habitats, including fields, forests, and forest edges. Most survey respondents considered long-tailed weasels to be relatively scarce. The information we have collected more than doubles the number of counties where long-tailed weasels are known to exist, and indicates that this small carnivore is widely distributed throughout the state.