Rheumatic diseases affect about 20% of the population, leading to common symptoms such as joint problems, pain, fatigue, and stiffness. Loneliness is prevalent in individuals with rheumatic diseases. This could be due to not receiving social support and being stigmatized and invalidated, which might be most common in fibromyalgia, a rheumatic disease that lacks medical evidence. The aim of this study was to compare loneliness in distinct rheumatic diseases and to examine the association of loneliness with social support and invalidation. Participants were 927 patients with ankylosing spondylitis (n = 152), fibromyalgia (n = 341), osteoarthritis (n = 150), rheumatoid arthritis (n = 171), or systemic diseases (n = 113). They completed online questionnaires including an 11-point Likert scale assessing loneliness, the Illness Invalidation Inventory (3*1; Kool et al., 2010), and the Social Support Survey (SSS; De Boer, Wijker, Speelman, & De Haes, 1996; Sherbourne & Stewart, 1991). Patients with fibromyalgia experienced significantly more loneliness than patients with ankylosing spondylitis and patients with rheumatoid arthritis. Besides being younger, having lower education, and not working, in multiple regression analyses both lack of social support and invalidation were independently correlated with loneliness. This suggests that to decrease loneliness, therapeutic attention should be given to both increasing social support as well as decreasing invalidation in patients with rheumatic diseases, especially in patients with fibromyalgia.