People often self-identify as allies to the lesbian, gay, bisexual, and transgender (LGBT) community. This research examined on what basis LGBT individuals perceive others to be allies and documents the consequences of perceived allyship. Studies 1a (n = 40) and 1b (n = 69) collected open-ended descriptions of allyship provided by LGBT participants. Coding of the responses suggested multiple components to being an ally: (a) being nonprejudiced toward the group, (b) taking action against discrimination and inequality, and (c) having humility about one's perspective in discussions about LGBT issues. In Studies 2a (n = 161) and 2b (n = 319, with nationally representative characteristics), an allyship scale was developed and validated for general and specific relational contexts, respectively. Study 2b also showed that LGBT individuals' perceptions of close others' allyship were positively associated with their own well-being and relationship quality with the close other. Study 3, an experiment, demonstrated that nonprejudice and action had an interactive effect on perceived allyship, such that action increased perceived allyship more when prejudice was low (vs. high). Study 4 was a weekly experience study of LGBT participants and an outgroup roommate. Perceiving one's roommate to be a good ally predicted higher self-esteem, greater subjective well-being, and better relationship quality with the roommate, both between and within participants. Furthermore, perceived allyship in 1 week was associated with increases in LGBT individuals' mental health and relationship quality with the roommate the following week. This research advances knowledge about what allyship means to LGBT individuals and identifies intra- and interpersonal benefits of allyship. (PsycInfo Database Record (c) 2023 APA, all rights reserved).