BACKGROUND: Loneliness is associated with adverse psychological and physical outcomes. However, little is known about the factors contributing to loneliness in autistic adults. This study aimed to quantitatively compare levels and predictors of loneliness in autistic and nonautistic adults, and then contextualize these findings by thematically analyzing responses to open-ended questions on autistic adults' socialization experiences. METHODS: We obtained data from the Cooperative Research Centre for Living with Autism (Autism CRC) Australian Longitudinal Study of Adults with Autism (ALSAA). The sample comprised 220 autistic adults (age mean [] = 41.9 years, standard deviation [SD] = 12.24) and 146 nonautistic adults (age  = 43.7 years, SD = 13.49). We measured loneliness with the University of California, Los Angeles (UCLA) loneliness scale (ULS-8). We compared the findings between these two groups of adults. Through regression models, we investigated associations between loneliness and demographics, autistic traits, social support, depression, anxiety, and self-efficacy. We adapted these determinants from De Jong-Gierveld's model of loneliness. In addition, we conducted an inductive thematic analysis of autistic participants' open-ended responses about their socialization. We used an inclusive approach utilizing an advisory panel of autistic adults in study design and interpretations. RESULTS: Autistic adults scored significantly higher on the ULS-8 than nonautistic adults ( < 0.001). The presence of autism contributed the greatest variance in the loneliness score ( = 8.11, 95% confidence interval [6.98-9.23],  < 0.001,  = 0.38). The autism quotient subdomains of social skills and dissatisfaction with social support were associated with greater loneliness in both autistic and nonautistic groups ( < 0.05). The thematic analysis contextualized the interpretation of quantitative findings, specifically regarding perceived loneliness and difficulties with social interaction. Satisfaction and perceptions of socialization were widely variable and both of which were shaped by experiences. Environmental factors, noise in particular, as well as social communication difficulties and past negative experiences seemed to be the barriers to socialization. CONCLUSIONS: Autistic adults have reported higher levels of loneliness. Variables associated with loneliness in both groups were dissatisfaction with social support and the autism quotient subdomain of social skills. The subjectivity of perceived loneliness, views about socialization, and their implications for social support in autistic adults warrant further study.