Introduction: Social dysfunction is a core feature of first-episode psychosis (FEP) that broadly impacts effective interpersonal interaction. FEP often begins during emerging adulthood (i.e., the late teenage years into the early adult years), which is a critical period for development and maintenance of social relationships. Social support positively augments the early course of psychosis, yet little is known about factors that influence social support-seeking behavior in FEP. Notably, defeatist performance beliefs are prominent in psychotic disorders and interfere with goal-directed behavior. Though individuals with psychosis have elevated levels of negative social beliefs related to feeling like a burden to others (i.e., perceived burdensomeness; PB) and disconnection from others (i.e., thwarted belongingness; TB), how these beliefs impact perceptions of social support or support-seeking behavior is unknown. Methods: 42 participants with first-episode psychosis completed laboratory -based assessments of social relationships and negative social beliefs, followed by 4 weeks of ecological momentary assessment (EMA) to assess real-world engagement in social support-seeking behavior. Correlational analyses and generalized linear mixed-effect models were used to examine the contribution of PB and TB to both laboratory-based and real-world social perception and behavior. Results: Perceived burdensomeness (PB) and thwarted belongingness (TB) were negatively correlated with perceived social support. EMA revealed a within-person effect whereby individuals were most likely to seek social support when experiencing more negative affect than was typical of them. Finally, higher levels of baseline PB and TB were predictive of less social support-seeking behavior in everyday life. Conclusion: Negative interpersonal beliefs (i.e., PB and TB) are related to both perceptions of social support and real-world social support-seeking behavior in FEP. Beliefs about burdening others and individual-level variation in negative affect may be particularly impactful on social behavior. Collectively, our results highlight the relevance of interpersonal beliefs in predicting social behavior in FEP.