Suicide is a significant public health concern school-aged youth. Although a growing body of literature has documented the association between cyberbullying and suicidal ideation, and the mediating effect of internalizing symptoms, there is no research to date examining the impact of witnessing cyberbullying and suicidal ideation. To address this gap, we conducted a cross-sectional study with middle school students (N = 130). Students completed questionnaires assessing witnessing cyberbullying, witnessing school bullying, depression, anxiety, and suicidal ideation. We used structural equation modeling to test a mediational model in which we hypothesized internalizing symptoms would mediate the unique relationship between witnessing cyberbullying and suicidal ideation (i.e., controlling for witnessing school bullying). Results supported the mediational model, indicating the frequency of witnessing cyberbullying was positively related to internalizing symptoms, which in turn were related to higher levels of suicidal ideation. Findings suggest the importance of providing programs to support middle school students who witness cyberbullying to reduce the mental health risks (i.e., internalizing symptoms and suicidal ideation) associated with being a cyberbullying bystander.