Youth are disproportionately affected by substance use and associated sexual risk behaviors, increasing STI and HIV susceptibility. This study analyzed the interplay between alcohol/drug use before sex (ABS/DBS), perceived sex approval (e.g., perceived familial or peer approval in engaging in sex), and HIV/STI risk perception among youth aged 13-21 (n = 150). We assessed how these factors influenced condomless vaginal/anal sex (VAS) and oral sex practices. Results showed significant interactions between perceived sex approval and lifetime ABS for lifetime condomless VAS, and between HIV/STI risk perception and lifetime ABS for current condomless oral sex. These findings underline the influence of perceived sex approval and HIV/STI risk perception on youth's sexual behaviors. Implications for targeted interventions are discussed.