To investigate the prevalence of loneliness among patients with IA with a specific focus on the associations with disease activity and impact. We used data from a Danish cross-sectional survey comprising information on socio-demographics, mental health status, and social contacts among 12 713 patients with IA (rheumatoid arthritis (RA)/psoriatic arthritis (PsA)/axial spondylarthritis (axSpA)). Data were linked to the DANBIO Rheumatology Registry and the National Patient Registry. Loneliness was measured by asking: "Are you ever alone, although you would prefer to be together with others?". Association with disease activity and disease impact (Patient Global Assessment, pain, fatigue, physical function) was estimated using multivariable logistic regression (age, sex, cohabitation status, educational level, mental health status (depression, anxiety), and co-morbidity). Approximately one-third reported loneliness. Prevalence was lowest for patients with RA (31.6% (95%CI: 30.5; 32.6)) compared with PsA and axSpA (36.0 (34.0; 38.0)) and (36.3 (34.1; 38.4), respectively). It was highest among axSpA patients reporting high levels of depression (66.2% (60.0; 72.8)). A positive association was observed between loneliness and disease activity. For disease impact, prevalence estimates were between 40-60% when patients experienced high levels of pain, fatigue, low levels of physical function, and high Patient Global Assessment. Loneliness was highly prevalent in IA and associated with disease activity and impact. Therefore, loneliness is an important target for future mental health interventions in IA.