ABSTRACT The gateway hypothesis posits that marijuana use initiation may lead to neurological changes predisposing individuals to escalation to harder drug use later on. However, there is limited research, which has examined marijuana as a gateway drug leading to hallucinogen use and examination of mechanisms linking the two drug use behaviors. This study examined marijuana use as a predictor of hallucinogen use and tested for mediation via social, affective, and cognitive mechanisms. The Pathways to Desistance data were analyzed. This is a longitudinal dataset comprised a sample of 1,354 justice-involved youth. Path analyses were used to test for direct and indirect effects of interest. Findings indicated that elevated marijuana use frequency prior to baseline was a significant predictor of increased log-odds of later hallucinogen use. None of sensation-seeking, impulse control, nor deviant peer association significantly mediated this relationship. Ever having used marijuana prior to baseline was not significantly related to later hallucinogen use risk and no mediation effects were identified for this independent variable. Results indicated that higher frequency marijuana use may result in gateway effects for later hallucinogen use. Both marijuana use variables were also statistically significant predictors of increased sensation-seeking, diminished impulse control, and greater deviant peer association.