As a common experience for sexual minority individuals, sexual orientation concealment has complex implications. Self-determination theory (SDT) and previous studies suggest a mediation path where hiding sexual orientation is associated with well-being through eroding lesbian, gay, bisexual-specific authenticity and inducing loneliness. However, this relationship has rarely been examined using longitudinal data. This study also built on minority stress theory to determine whether the psychological process of concealment operates differently as a function of perceived acceptance in different contexts. A community sample of 636 sexual minority individuals in Hong Kong (48.3% men, 50.5% women, and 1.3% transgender and nonbinary individuals; mean age = 25.36 years) was recruited through lesbian, gay, bisexual, and transgender (LGBT) social media, nongovernmental organizations, and social venues to complete a baseline and 1-year follow-up survey. The results showed that loneliness mediated the relationship between concealment and well-being. Moreover, the mediation path from concealment to well-being via loneliness was moderated by the perceived LGBT-friendliness of the family. The negative association between concealment and well-being was only significant when the family was perceived as supportive of sexual and gender minorities. The study findings elucidated the complex mechanisms involved in the adverse outcomes of sexual orientation concealment and highlighted the importance of loneliness and family context when working with sexual minority clients on their concerns over sexual orientation concealment. (PsycInfo Database Record (c) 2022 APA, all rights reserved).