Impostorism is a psychological phenomenon characterized by persistent self-doubt and a fear of being exposed as an intellectual fraud, despite objective records of success. This phenomenon was first observed among high achieving women. The current study investigated impostorism among graduate women in science, technology, engineering, and mathematics (STEM) programs, a group that has historically been underrepresented and often subjected to negative stereotypes and discriminatory treatment, increasing their vulnerability to impostor experiences. A diverse group of 80 graduate women in STEM from various institutions in the U.S. participated in this study. Results demonstrated a high prevalence of impostorism with 97.5% reporting at least a moderate level of impostor experiences. Furthermore, a series of simple linear regression analyses revealed that impostorism predicted poorer mental health as well as greater burnout and dropout consideration, supporting the study hypotheses. Given that impostorism is experienced by high achieving individuals, impostorism was not expected to predict academic achievements. Supporting this hypothesis, impostorism did not predict academic achievements, as measured by the number of publications and graduate GPA. Implications and future directions are discussed.