<p>Employee organizational socialization has been increasingly emphasized as an essential process benefiting the environment and organizational sustainability. Nevertheless, both scholars and practitioners need a fuller nuanced understanding of the antecedents and boundary conditions of organizational socialization. Drawing on social learning theory, we examined the effect of humble leadership and employee humility on employee adaptive performance and the mediating role of proactive socialization. The synergy effects of humble leadership and employee humility were also evaluated. A two-wave data survey was conducted from a sample of 297 employees. Results indicated that both humble leadership and employee humility have positive effects on employee adaptive performance. Employee proactive socialization mediated the relationship between humble leadership and employee adaptive performance, as well as mediated the relationship between employee humility and adaptive performance. In addition, there is a synergistic effect between humble leadership and employee humility in predicting employee proactive socialization and adaptive performance. The theoretical and practical implications for humble leadership, employee humility, proactive socialization, and adaptive performance were discussed.</p>