Abstract Social ostracism has been consistently linked to aggressive behavior. However, whether this relationship exists in cyberspace and the variables that might reduce cyber aggression are largely unexplored. This presents an obstacle to fully understanding the origins of cyber aggression and to the development of effective prevention and treatment strategies. We conducted two studies to test the hypothesis that dispositional and experimentally primed attachment security would reduce cyber aggression after cyber ostracism. In Study 1, undergraduates were exposed to a cyber ostracism or inclusion condition in an online game, “Cyberball,” and then were given a chance to score the creative painting works of two other players in Cyberball. After cyber ostracism, participants with relatively high attachment avoidance (i.e., lower attachment security) showed higher aggressiveness than did participants with low avoidance (i.e., higher attachment security). In Study 2, all procedures were as the same as in Study 1 except for that, after playing the Cyberball game, participants received a priming manipulation for attachment security. After cyber ostracism, compared to those who were neutrally primed, participants who recalled an attachment figure exhibited less cyber aggression. These results might help to design strategies for effective prevention of aggressive behaviors in cyberspace.