Many studies have documented associations of substance use with aggression perpetration and aggression victimization; however, little is known about the co-occurrence of these problem behaviors within the same day in college students. The present study investigated whether substance use and aggression increase the likelihood of each other and whether attitudes justifying aggression strengthen those associations. College student participants (N = 378, 32% males) self-selected into an online study in which they reported on 2 days of alcohol/drug use and on aggression perpetration and victimization (including physical, psychological and electronic aggression, and sexual coercion) with friends and dating partners. Using regression to test for nonequivalence of predictor and outcome variables, we found bidirectional effects for males only. Males’ substance use was associated with an increased likelihood on the same day of aggression perpetration and of aggression victimization; males’ aggression perpetration and aggression victimization were associated with an increased likelihood of substance use on the same day. Females did not show significant contingencies between substance use and aggression in either direction. Males’ attitudes justifying male-to-female aggression were associated with their aggression perpetration and victimization and their justification of female-to-male aggression strengthened the link between substance use and aggression perpetration. With interpersonal aggression and substance use being significant problems on college campuses, many colleges offer separate preventive intervention programs aimed at these public health challenges; this study suggests possible benefits of an integrated approach that addresses connections between alcohol/drug use and aggression.