Organizations are increasingly augmenting employee tasks with artificial intelligence (AI). Whereas the existing literature predominantly describes AI augmentation as a positive practice that enhances employee capabilities and task performance, we attempt to challenge this consensus by exploring its dark side. Drawing upon the theory of impostor phenomenon, we posit that workplace AI augmentation can evoke workplace impostor thoughts in employees and in turn decrease their knowledge sharing and interpersonal citizenship behavior. Moreover, we suggest that these effects can be more pronounced when employees have a higher level of intrinsic motivation. We tested our hypothetical model with three experiments and a longitudinal survey. This research challenges the consensus in the literature by revealing the dark side of both AI augmentation and intrinsic motivation, and offers important contributions to research on impostor phenomenon.