Impostorism and knowledge-hiding behaviors negatively impact employees and organizational performance. This study examines the association between impostor leaders and knowledge hiding (evasive hiding, playing dumb, and rationalized hiding). Attachment avoidance is discussed as a mediator between impostor leaders and knowledge-hiding. For quantitative analyses, this study collected the data from 429 individuals with two time lags by sharing the survey instrument link on different organizations\' randomly selected official media pages. After obtaining approval from the administrators of these pages, leaders and subordinates from these organizations were asked to participate in the study. The partial least squares structural equation modeling method is employed with Smartpls-4 software for data analyses. The findings indicate that impostor leaders promote knowledge hiding in subordinates. However, impostor leaders highly promote rationalized hiding behavior in subordinates. Attachment avoidance mediates the relationship between the impostor leader and knowledge-hiding behaviors. However, the highest mediation relationship exists between an impostor leader and playing dumb behavior in subordinates. This study strengthens the generalizability of the social exchange theory. The implications mentioned in this study are beneficial in understanding and dealing with the Impostorism and knowledge-hiding phenomena.