Cyberbullying is a significant problem among school-aged youth. Although a growing body of literature has documented the association between cyberbullying victimization and mental health risks, there is limited research examining the impact of witnessing cyberbullying, particularly among elementary school students. To address this gap, we conducted a cross-sectional study with elementary school students (N = 122). Students completed questionnaires assessing witnessing cyberbullying, witnessing school bullying, bullying victimization, depressive symptoms, and social anxiety. Regression analyses revealed that witnessing cyberbullying was positively associated with depressive symptoms ([beta] = 0.25, p < 0.04) and social anxiety ([beta] = 0.30, p < 0.01), even after controlling for frequency of witnessing school bullying and bullying victimization. Further, the moderating effect of witnessing school bullying was significant for depressive symptoms ([beta] = -0.44, p < 0.001) and social anxiety ([beta] = -0.31, p < 0.01), such that students who witnessed cyberbullying only reported the highest level of depressive symptoms and social anxiety. The moderating effect of bullying victimization was not significant. Findings suggest the importance of providing programs to support elementary school students who witness cyberbullying to reduce the mental health risks associated with being a cyberbullying bystander.