Casual sexual relationships and experiences (CSREs) are still considered to be detrimental to the psychological well-being of youth even though findings remain inconclusive. Most studies have focused on emerging adulthood. Using a prospective design based on a representative sample of high school students in the province of Québec, we measured sexually active adolescents' (N = 2,304) psychological well-being six months after engaging in these relationships while controlling for prior well-being. We analyzed two forms of CSREs, friends with benefits (FWB) and one-night stand (ONS) relationships, as well as levels of sexual intimacy. The results show that CSREs had a small impact (small effect sizes) on subsequent psychological well-being, especially among girls; FWB relationships involving penetrative contact increased girls' psychological distress and both their alcohol and drug consumption. ONSs including sexual touching increased girls' psychological distress and their drug use. None of the CSREs influenced boys' psychological well-being. The findings underscored the importance of using caution when arguing that CSREs are detrimental or harmless to the psychological well-being of adolescents. The results also highlight the importance of taking into account gender and forms of CSREs in prevention and health interventions.