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 (LGBTQIAϩ) communities. The relationships between loneliness, mental health, social support, belongingness, and quality of life in a sample of LGBTQIAϩ (n ϭ 238) and non-LGBTQIAϩ (n ϭ 270) adults aged 18 -73 years (N ϭ 508) were examined. Overall, LGBTQIAϩ adults experienced higher levels of loneliness, depression, and social anxiety than the non-LGBTQIAϩ comparison group. LGBTQIAϩ adults also reported lower perceived social support and were at higher risk of social isolation than the non-LGBTQIAϩ group. Thus, LGBTQIAϩ adults may be at greater risk of loneliness and social isolation than has previously been recognized. Public Policy Relevance StatementThe lesbian, gay, bisexual, transgender, queer, intersex, asexual, and other sexual orientation and gender identity diverse (LBGTQIAϩ) community often experience problematic mental health concerns, fewer social supports, and increased social isolation compared with non-LGBTQIAϩ communities. Few studies have investigated whether loneliness, a known correlate of mental and physical health conditions, is problematic in the LGBTQIAϩ community. This study suggests that identifying as LGBTQIAϩ has negative implications on loneliness, mental health, and social supports compared with a heteronormative sample.