A growing body of research has found that perceptions of social–emotional support from school personnel and lesbian, gay, bisexual, transgender, and queer (LGBTQ) in-school resources (e.g., gender and sexuality alliances, inclusive curricula, inclusive sexual education, presence of affirming adults) contribute to sexual and gender diverse youth’s (SGDY) positive development. However, no research has investigated how support from school personnel and LGBTQ in-school resources might jointly modify the associations between SGDY’s experiences with sexual orientation and gender identity (identity)-based harassment and mental health outcomes. Thus, the current study examined how school personnel support and LGBTQ in-school resources, together, moderated the association between identity-based harassment, depressive symptoms, and anxiety symptoms among a national sample of SGDY (N = 13,500, Mage = 15.50, SD = 1.34). Multigroup path analysis revealed that for SGDY in middle school (i.e., 6–8) and high school grade levels (i.e., 9–12), school personnel support and LGBTQ in-school resources jointly moderated the association between identity-based harassment and depressive symptoms. Among SGDY who reported high levels of harassment, support from school personnel buffered the association between identity-based harassment and depressive symptoms. However, at low levels of harassment, it was a combination of high support from school personnel and LGBTQ in-school resources that was linked to the lowest levels of depressive symptoms. SGDY reported more depressive symptoms when they perceived low levels of support from school personnel, regardless of the concentration of LGBTQ in-school resources. (PsycInfo Database Record (c) 2025 APA, all rights reserved)