Eating disorders (EDs) are associated with high mortality rates from suicide. Empirical tests of the Interpersonal-Psychological Theory of Suicide (IPTS) have provided preliminary cross-sectional support for its application to individuals with EDs. Because IPTS seeks to predict development and changes in suicidal ideation (SI), longitudinal investigations are ideal. The purpose of this study was to conduct cross-sectional and longitudinal mediational tests of the effect of ED psychopathology on SI as explained by perceived burdensomeness, thwarted belongingness, and hopelessness. Participants were undergraduate students (N = 738) who completed self-report measures of ED symptoms and IPTS variables at up to three time points across 10 weeks. Multiple mediation analyses were conducted on cross-sectional and longitudinal data. Cross-sectional analyses indicate mostly consistent findings with existing literature; however, results from the longitudinal analyses failed to identify any mediational effects of ED psychopathology on SI. These differences emphasize the importance of empirical tests in both cross-sectional and longitudinal data. Given the inconsistent results, the utility of IPTS features in explaining the association between ED psychopathology and SI is unclear. Future studies should seek to replicate these findings using other methods of measurement across time (e.g., ecological momentary assessment) and within clinical ED samples.