This study aimed to characterize aggressive behaviors in autistic youths and to identify the social environment variables most strongly linked with aggression in this clinical population. Participants were 2142 caregivers of autistic youths (ages 6.0–17.9) recruited from autism research centers across the United States. Caregivers completed self‐report and behavior rating inventories that assessed both verbal and physical aggression as well as characteristics of the individual youths (sleep quality, gastrointestinal [GI] symptoms, and autism characteristics) and their families (caregiver stress, global family functioning, and sibling relations), peers (emotional bonding, number of friends), schools (academic functioning), and neighborhoods (perceived community safety). We used descriptive analyses to identify which aggressive acts were most common among autistic youths, and we performed bivariate correlations and multiple linear regression analyses to determine which characteristics of the youths and their social environments were most strongly linked with youth aggression. Verbally aggressive youth behaviors were endorsed by caregivers most frequently. Youth age and sex were not associated with verbal or physical aggression. A combination of youth and social environment characteristics accounted for 42.6% of the variance in verbal aggression and 26.0% of the variance in physical aggression. Thus, those characteristics most strongly linked with verbal and physical aggression were strained sibling relations, caregiver stress, youth sleep problems, and youth repetitive and restrictive behaviors. Viewed together, the results suggest that aggressive behaviors in autistic youths are associated with multiple characteristics pertaining to the individual youths and their immediate social environments. Implications for treatment and research are discussed.