The existence of loneliness stigma is often mentioned, with little explanation of what this is, what its impact might be, and how it might be addressed. Drawing on conceptualizations of stigma, we define loneliness stigma, explain its roots and functions, and summarize some of its various manifestations. We additionally describe some of the evidence for the existence of a stigma associated with loneliness, from the perspective of the stigmatizer (enacted stigma and courtesy stigma) and from the perspective of the stigmatized (perceived community stigma, perceived courtesy stigma, and self-stigma). In doing so, we highlight some of the ways in which public, policy, and academic discourses unwittingly contribute to the stigmatization of loneliness. To underline why we should care about loneliness stigma, we detail existing evidence for its impacts on wellbeing, over and above the impact of loneliness, as well as a barrier to overcoming loneliness. Finally, we discuss two of the ways in which loneliness stigma can be prevented or reduced: Through interventions directly aimed at reducing stigma and by ensuring that interventions to reduce loneliness do not inadvertently contribute to stigmatizing it.