Newcomers are often referred to as the “new blood” because they represent a source of fresh, unique, task-relevant knowledge that potentially adds value to organizations. In this research, I focus on newcomer proactive knowledge sharing with coworkers and investigate how it impacts the transition of newcomers from outsiders to insiders. Integrating attribution theory and the status characteristics theory, I propose that newcomer proactive knowledge sharing with coworkers triggers coworkers’ ambivalent attributions (i.e., perceiving it to be driven simultaneously by newcomers’ prosocial and status-striving motives). Furthermore, the ambivalent attributions affect the extent to which coworkers provide socialization support and utilize the newcomer’s knowledge, eventually exerting different influences on the newcomer’s inclusion perceptions. The results of a multi-wave (i.e., four waves) and multi-source (i.e., survey data from newcomers and coworkers) longitudinal study based on 336 newcomers in a large technology company support the proposed serial mediating relationships between newcomer proactive knowledge sharing with coworkers and their inclusion perceptions via coworkers’ ambivalent attributions and behavioral reactions. The data also demonstrates that leader encouragement of learning is a viable leader strategy that makes coworkers more likely to interpret newcomer proactive knowledge sharing is driven by prosocial motives. This research has significant implications both theoretically and practically. From a theoretical perspective, it advances our understanding of newcomer socialization, knowledge sharing, and workplace inclusion. From a practical perspective, it helps newcomers better navigate the process of knowledge sharing by illuminating potential social consequences. Practitioners can leverage these insights to create more inclusive onboarding experiences for new employees.