<p>Substance use, particularly cannabis and alcohol, is increasingly prevalent among young adults in the U.S. due to cultural shifts and greater social acceptance. At the same time, sexual agency, defined as the ability to communicate, negotiate, and make informed decisions about one’s sexual needs, is essential for sexual health and well-being. However, limited research exists on how substance use affects sexual agency, especially in this age group. This cross-sectional study examined the relationship between cannabis and alcohol use and sexual agency in young adults aged 18–20 years (<i>n</i> = 857). Participants completed an online survey assessing sexual agency via the Sexual Agency Questionnaire and self-reported substance use. Data were analyzed using zero-inflated Poisson regression models. Frequent cannabis use was linked to lower levels of sexual communication, increased risk-taking behaviors, and reduced boundary-setting. Alcohol use had mixed effects: while it was associated with greater sexual enjoyment and comfort discussing sexuality, it also correlated with riskier sexual behaviors and reduced communication of consent, especially with binge drinking. These findings suggest that cannabis may impair cognitive functions necessary for effective sexual communication, while alcohol use can both enhance confidence and increase risks by lowering protective behaviors. The study highlights the need for public health interventions that address both substance use and sexual agency, focusing on educating young adults about the risks of substance use in sexual contexts and promoting safer sexual practices.</p>