There has been little research attention given to how Gay-Straight Alliances might mitigate mental health consequences associated with experiencing homophobic cyberbullying. To address this gap in knowledge, the purpose of this one-year longitudinal study was to investigate the moderating effect of perceived social support from Gay-Straight Alliances in the relationships among homophobic cyberbullying victimization and bystanding and depressive and anxiety symptoms among 466 LGBTQIA adolescents (Mage = 15.76; 52% female). The findings revealed that perceived social support was related negatively to homophobic cyberbullying involvement and depressive and anxiety symptoms. Homophobic cyberbullying involvement was related positively to depressive and anxiety symptoms. High perceived social support buffered against the depressive and anxiety symptoms resulting from homophobic victimization and bystanding among LGBTQIA adolescents but low levels and average levels did not moderate these associations. These findings highlight the importance of expanding Gay-Straight Alliances in schools.