We examined whether romantic relationship involvement, a well-established protective factor against mental health problems among heterosexual adults, is also protective for sexual and gender minority emerging adults assigned female at birth (SGM-AFAB), a group at high risk for mental health issues. Using cross-sectional data from a community sample of 222 SGM-AFAB ages 18–20 years, we assessed associations between current relationship involvement and five mental health variables: depressive symptoms, anxiety symptoms, alcohol use problems, cannabis use problems, and illicit drug use. There were no differences by romantic involvement in problematic cannabis use or other illicit drug use. Overall, participants in a relationship reported fewer depressive symptoms, fewer anxiety symptoms, and less problematic alcohol use than participants who were single. Some associations differed, however, by participant gender identity, sexual orientation identity, and partner gender. Specifically, relationship involvement was associated with fewer depressive and anxiety symptoms for cisgender female participants ( n = 154) but not for gender minority participants ( n = 68) and for lesbian participants ( n = 38) but not for bisexual/pansexual participants ( n = 134) or those with other sexual orientation identities ( n = 50). Participants romantically involved with a cisgender female partner ( n = 43) had fewer depressive and anxiety symptoms than single participants ( n = 100), those with a cisgender male partner ( n = 56), and those with a gender minority partner (n = 23). Together, these findings suggest that romantic involvement may promote mental health for many, but not all, SGM young adults, highlighting the importance of attending to differences among SGM subgroups in research and efforts to reduce mental health and substance use disparities.