Impostor syndrome, also known as impostor phenomenon or impostorism, is a psychological occurrence in which people doubt their skills, talents, or accomplishments and have a persistent internalized fear of being exposed as frauds. Despite external evidence of their competence, those experiencing this phenomenon do not believe they deserve their success or luck. They may incorrectly attribute it to the Matthew effect or the Dunning–Kruger effect, or they may think that they are deceiving others because they feel as if they are not as intelligent as they outwardly portray themselves to be. Impostor syndrome can stem from and result in strained personal relationships and can hinder individuals from achieving their full potential in their fields of interest.

When impostor syndrome was first conceptualized, it was viewed as a phenomenon that was common among high-achieving women. Further research has shown that it affects both men and women, in the collective sense that the proportion affected are more or less equally distributed among the genders. Individuals with impostor syndrome often have corresponding mental health issues, which may be treated with psychological interventions, though the phenomenon is not a formal mental disorder.