Persons with mental illness are highly susceptible to experiencing public, anticipated, and internalized mental illness stigma, which can manifest in a myriad of ways to negatively affect quality of life, treatment outcomes, and attainment of personally meaningful goals. Research also shows that the impostor phenomenon and rumination are prominent cognitive processes experienced by persons with mental illness in unique ways. This is the first study to examine the relationship among all three psychological constructs. Survey data were collected from an online sample of 90 participants with one or more professionally diagnosed mental illnesses. Mental illness stigma was assessed with the Inventory of Stigmatizing Experiences and the Internalized Stigma of Mental Illness scale. The impostor phenomenon was assessed with the Leary Impostorism Scale, and rumination was assessed with the Scott-McIntosh Rumination Inventory. Correlational analyses revealed that public, anticipated, and internalized mental illness stigma scores were significantly correlated with both impostor phenomenon and rumination scores. Impostor phenomenon scores were significantly correlated with rumination scores as well. Moderated multiple regression models revealed that the impostor phenomenon moderated the relationship between public mental illness stigma and the distraction dimension of rumination. These results add to the growing body of evidence documenting the nuanced ways in which stigma interacts with other psychological factors to impact persons with mental illness.