Gender-Sexuality Alliances (GSAs) and inclusive school policies and practices that affirm youth with minoritized sexual orientations or gender identities (e.g., lesbian, gay, bisexual, transgender, queer youth; LGBTQ+ youth) are two sources of support for LGBTQ+ youth that could promote school belonging. The current study tested a three-level multilevel model in which youth's GSA experiences and the degree to which their schools implemented LGBTQ+ inclusive policies and practices predicted their school belonging over a six-month period. Participants included 627 youth (87% LGBQ+ youth, 45% transgender or nonbinary youth, 48% youth of color) ages 11-22 (M[subscript age] = 15.13) in 51 GSAs. At the within-individual level, youth reported greater school belonging on occasions following months when they felt their peers and advisors were more responsive to their needs and when they had taken on more leadership in the GSA. At the between-individual level, youth who generally felt their peers were more responsive over the study period reported greater school belonging than others. At the between-GSA level, GSA members in schools that more thoroughly implemented LGBTQ+ inclusive policies and practices reported greater school belonging over the study period. These findings underscore the relevance of GSAs and inclusive policies and practices in establishing welcoming school environments. [This is the online first version of an article in "Journal of Youth and Adolescence."]