Background: Bullying has regularly been shown to be associated with suicide ideation. However, so far, only a few studies have examined the mechanisms underlying the relationship between bullying and suicide ideation within a theory of suicide. Aim: The aim of the present study was to investigate whether thwarted belongingness and perceived burdensomeness mediate the association between bullying and suicide ideation. Method: A total of 267 outpatients (63.3% female; M-age = 37.52, SDage = 12.80) completed online measures of bullying, suicide ideation, thwarted belongingness, and perceived burdensomeness. Results: Thwarted belongingness as well as perceived burdensomeness fully mediated the association between bullying and suicide ideation - controlling for gender and age. Limitations: Given the cross-sectional nature of the data, only hypothetical conclusions on causality can be drawn. Conclusion: Our results support the hypothesized and theory-derived relations between bullying, thwarted belongingness, perceived burdensomeness, and suicide ideation. In anti-bullying programs it seems to be of particular importance to counter impressions of thwarted belongingness and perceived burdensomeness.