Borderline personality disorder (BPD) is characterized by emotional reactivity and interpersonal sensitivity, including greater emotional and cognitive reactivity to social rejection than controls. However, existing social rejection paradigms are not relevant to the particular social contexts that are increasingly relevant for adolescents and young adults (i.e., social media and online settings). This study examined emotional, parasympathetic, and cognitive responses to a novel online group chat social interaction task among emerging adults (18-24 years old) with a range of BPD pathology. Consistent with hypotheses, results revealed greater hostility and upset feelings, worse mood, greater threat to social needs, and greater respiratory sinus arrhythmia withdrawal in response to this task among participants in the rejection versus inclusion condition. Less support was found for the hypothesized relation of BPD pathology to greater reactivity to online social rejection. Specifically, although results revealed positive relations of BPD pathology with both negative mood in general and hostility in particular, in response to the task among participants in the rejection (but not inclusion) condition, the effect sizes associated with these findings were small, and the other hypothesized interactions of BPD pathology and group chat task condition were not significant. Conversely, results revealed associations of BPD pathology with greater threat to social needs and lower positive mood in response to the group chat task, regardless of condition. Results provide preliminary support for the utility of this novel online group chat rejection task for eliciting negative emotional, parasympathetic, and cognitive reactions in emerging adults with a range of BPD pathology. (PsycInfo Database Record (c) 2020 APA, all rights reserved).