This survey-based study sought to measure the experience of impostor phenomenon among library personnel supporting scholarly communications in academic libraries in the United States. Additionally, the survey sought to assess confidence levels in key, professionally defined competencies and the factors most significantly affecting those confidence levels. Results indicated that, on average, scholarly communications librarians experience impostor phenomenon more frequently and intensely than academic librarians more broadly. The length of time spent working in libraries was negatively correlated with levels of impostor phenomenon, as were hours spent in specialized continuing education activities and number of research publications. Implications for improving training and mentoring opportunities to decrease impostor phenomenon are discussed.