Loneliness is a growing public health concern that is associated with poor mental health (e.g., social anxiety, depression) and increased physical health problems (e.g., cardiovascular disease, sleep disturbances). Socially vulnerable groups such as the elderly, migrants, and asylum seekers are more susceptible to the effects of loneliness. We examined loneliness severity in lesbian, gay, bisexual, transgender, queer, intersex, asexual, and other sexual orientation and gender identity diverse (LGBTQIA+) communities. The relationships between loneliness, mental health, social support, belongingness, and quality of life in a sample of LGBTQIA+ (n = 238) and non-LGBTQIA+ (n = 270) adults aged 18-73 years (N = 508) were examined. Overall, LGBTQIA+ adults experienced higher levels of loneliness, depression, and social anxiety than the non-LGBTQIA+ comparison group. LGBTQIA+ adults also reported lower perceived social support and were at higher risk of social isolation than the non-LGBTQIA+ group. Thus, LGBTQIA+ adults may be at greater risk of loneliness and social isolation than has previously been recognized. (PsycInfo Database Record (c) 2021 APA, all rights reserved).