Although most studies have shown that newcomers benefit from proactive behaviors, these behaviors are not always viewed positively by colleagues, resulting in negative consequences for newcomers. Drawing on uncertainty reduction and social cognitive theories, we contend that newcomer proactive behaviors are viewed positively by competent leaders and peers but negatively by those with low competence. Further, we argue that newcomer proactive behaviors impact leader and peer threat perceptions, affecting subsequent workplace relationships, which in turn influence newcomer voluntary turnover. We empirically test our hypotheses in a three-time multisource study, utilizing a sample of 377 newcomers, 132 leaders, and 721 peers. Supporting our hypotheses, when leaders and peers are less competent, newcomer proactive behaviors, through impacting leader and peer threat perceptions, result in lower quality relationships with leaders and peers. These cascading effects positively correlate with newcomer voluntary turnover. Conversely, opposite effects arise for more competent leaders and peers. Implications of how newcomer proactive behaviors impact workplace relationships and turnover are discussed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).