Aggressive behavior not only negatively affects an individual’s psycho-social adaptation but also undermines social harmony and stability. This study investigated the relationship between perceived crowdedness and aggression, examining the mediating role of relative deprivation and the moderating effects of upward social comparison and connectedness to nature based on the general strain theory and the I³ model. A cross-sectional design was employed, involving 848 participants (524 females, 61.8%) aged 20–75 (M = 33.15, SD = 6.83) in China. Structural equation modeling and bootstrapping procedures were used to test the hypothesized moderated mediation model. The results indicated that perceived crowdedness and upward social comparison positively predicted aggressive behavior, whereas connectedness to nature negatively predicted it. Relative deprivation mediated the relationship between perceived crowdedness and aggressive behavior. Only upward social comparison moderated the relationship between perceived crowdedness and relative deprivation, and connectedness to nature showed a marginally significant moderating effect on the relationship between perceived crowdedness and aggressive behavior. Unexpected trends are discussed, along with directions for future research.