We attend to the unexamined intersection between professional social network site (SNS) usage and imposter syndrome. Specifically, we provide the first examination of: do such sites cause imposter thoughts (“others think I am more competent than I think I am”); if so, why and when this happens, and what effect this has on well‐being and consumption‐related results. Supported by objective self‐focused attention theory and two online experiments, we show that professional SNS usage heightens professional self‐focused attention, triggering imposter thoughts. This results in negative emotions and consumption‐related effects. We further examine two boundary conditions, showing that effects are reduced for individuals high in narcissism or work centrality. From these findings, we extend the sociocognitive theorization of the imposter phenomenon by uncovering, first, context‐specific self‐focused attention as the reason “why” people feel imposter‐ish in particular circumstances and second, consumption‐related consequences. We further contribute imposter thoughts as a new alternative explanation for negative emotions experienced whilst using professional SNSs.