1. There is a high demand for biodiversity observation data to inform conservation and environmental policy, and citizen scientists generate the vast majority of terrestrial biodiversity observations. As this work is voluntary, many people assume that these data are openly available for use in conservation and scientific research. 2. Here, the openness of biodiversity observation data that are contributed to the Global Biodiversity Information Facility is examined by the data provider. Contrary to what many people assume, data sets from volunteers are among the most restrictive in how they can be used. 3. Policy implications. The assumption that voluntary data collection leads to data sharing does not recognize the wishes and motivations of those who collect data, nor does it respect the crucial contributions of these data to long-term monitoring of biodiversity trends. To improve data openness, citizen scientists should be recognized in ways that correspond with their motivations. Furthermore, organizations that manage these data should make their data sharing policies open and explicit.