Although a number of social--cognitive and contextual correlates of defending against bullying have been identified, research on the personality traits associated with defending have yielded weak and inconsistent results. The current study provided a novel examination as to whether a tendency toward social withdrawal is associated with less frequent defending and whether perceived injunctive norms for defending and aggression minimize the impact of social withdrawal on defending behaviors. A sample of 1,564 children (760 girls; M[subscript age] = 10.05; 55.0% White; 36.1% Black) were followed in the fall, winter, and spring of a school year. Defending was measured with self-reports and peer-reports. Social withdrawal was measured using teacher-reports. Perceived injunctive norms were estimated by calculating within-person correlations between participants' ratings of peers' popularity and defending and between peers' popularity and aggression. Results revealed that social withdrawal was associated with less peer-reported defending in the fall, and this effect was sustained over the school year. For boys, lower levels of social withdrawal in the fall were associated with less peer-reported defending when they viewed popular peers as unlikely to defend. A temporary (i.e., fall) association was found between viewing defenders as popular and self-reported defending, and children became less likely to self-report defending over the school year if they viewed popular children as aggressive. These findings underscore the need to examine how temperamental traits and perceived contextual norms cocontribute to bystanders' behavior when witnessing bullying.