Thwarted belongingness (TB) is among the most well-researched risk factors for suicidal ideation (SI). Yet, there is little research examining neural mechanisms underlying this construct. The present study used a novel social exclusion image set in concert with the late positive potential (LPP) to test the role of neural reactivity social exclusion in TB and SI. Participants (n = 243) were recruited based on elevated suicide risk and completed an emotional picture viewing task. The LPP was used to index neural reactivity to a novel set of social exclusion images and international affective picture system (IAPS) neutral, positive, and negative images. Greater TB predicted a larger social exclusion ΔLPP (social exclusion LPP relative to neutral LPP) and social exclusion LPP even when accounting for SI, biological sex, age, negative affect, and perceived burdensomeness. Moreover, the social exclusion LPP was uniquely related to TB beyond the LPP to other images. Last, a larger social exclusion LPP predicted greater SI via elevations in TB. Reactivity to social exclusion stimuli may play an important role in SI via elevated TB, but not perceived burdensomeness. Future research should further explore the neural mechanisms underlying other IPT constructs.