Violence among young adults is an increasing public health concern, especially in the context of nightlife, such as around nightclubs and bars. Nightlife is associated with alcohol, drugs, and increased violence, but little is known about personal and environmental factors related to physical aggression and sexual violence in nightclubs. This study aimed to determine personal and environmental risk factors for physical and sexual aggression in nightclubs in São Paulo, Brazil. Data were collected among nightclub patrons through use of a portal survey at the entrances and exits of 31 nightclubs. Men and women over 18 years old were systematically sampled while waiting in entrance lines. At the entrance, participants provided information about sociodemographic characteristics, drug use, alcohol use, and other risky behaviors during the prior 12 months. Upon exiting the nightclub, participants were asked about drug use, alcohol use, aggressive behaviors, and other risky behaviors that occurred while in the nightclub. Each participant was offered a breathalyzer test when entering and exiting the nightclub. Participants who used drugs in the nightclub, planned to have sex after leaving the club, or were younger in age were more likely to commit an act of physical aggression. Participants who attended nightclubs playing eclectic music, drank before arriving at the nightclub, and had elevated breath alcohol concentration at the entrance or exit were more likely to commit an act of sexual aggression. Study findings point to specific risk factors and can inform the development of social environmental prevention strategies to prevent physical and sexual aggression within nightclubs.