Key PointsQuestionDo transgender and gender-nonconforming (TGNC) and lesbian, gay, and bisexual (LGB) younger adults in China experience worse mental health and suicide risk than their cisgender heterosexual peers, and what are risk factors within TGNC and LGB groups?FindingsIn this cross-sectional survey of 89 342 younger adults in China, TGNC and cisgender LGB individuals had a higher prevalence of depression, anxiety, traumatic stress, nonsuicidal self-injury, and suicide risk compared with cisgender heterosexual individuals. Sexual minority women, gender nonbinary individuals, and those who reported peer bullying and assault and loneliness were particularly vulnerable.MeaningThe findings of this study suggest there is an imperative need to improve mental health and prevent suicide among Chinese LGB, transgender, or queer or questioning younger adults.