<p>In the realm of project management, leadership has long been a focal point of concern. However, the emergence and significance of informal leadership within the project context remain underexplored. This study, drawing on social exchange theory, hypothesizes the relationship between leader-member exchange (LMX) and the emergence of informal leadership, as well as the moderating effects of organizational complexity and ethical leadership. Using an experimental study (Study 1) and a multi-source survey (Study 2) with 198 participants from 68 project teams, this research demonstrates how informal leadership emergence acts as a mediator between LMX and project performance. Further analysis reveals that organizational complexity and ethical leadership strengthen the connection between LMX and informal leadership emergence. These findings offer a comprehensive explanation of how informal leadership emerges within project teams, identifying key conditions that facilitate it, and reveal its role and impact on project performance. By unveiling these dynamics, the study provides crucial insights into identifying and fostering informal leadership to enhance project performance.</p>