Objective This study examined the impact of social-cognitive constructs associated with increased suicide risk (i.e., perceived burdensomeness and thwarted belongingness) on relationship formation during real-time interactions of older adolescents with strangers in a dyadic context. Method Two hundred and seventy-four older adolescents (61.9% female, M age = 18.96 years) were paired with same-gender strangers and completed a laboratory-based interaction task. Both target participants and their partners answered questions about their interaction at three time points during the 45-min session. Structural equation modeling was used to test hypothesized models. Results Perceived burdensomeness and thwarted belongingness predicted interpersonal consequences during the interaction. In the first model, thwarted belongingness led targets to inaccurately perceive their partners’ assessments of the interaction. This misperception negatively impacted the participants’ own desires for friendship. In the second model, thwarted belongingness predicted decreases in targets’ desire to continue the conversation, whereas perceived burdensomeness predicted higher levels of desire to continue the conversation. For females, thwarted belongingness in particular was related to new relationship formation in these models. Overall, those who desired to continue talking were more likely to like their partner at the end of the conversation. Conclusions Results add to our understanding of how perceived burdensomeness and thwarted belongingness contribute to increased social alienation, which could serve to increase suicide risk.