Perfectionism is a transdiagnostic process that has been associated with a range of psychopathology and also with other transdiagnostic processes. We have previously shown that guided internet-based cognitive behavioural therapy (ICBT) can reduce symptoms of dysfunctional perfectionism, however, no impact was observed on symptoms of depression and anxiety. Here we explore the impact of guided ICBT for perfectionism on symptoms of other associated psychopathology, specifically obsessive-compulsive disorder (OCD) and eating disorders, and also on other associated transdiagnostic processes (self-esteem, intolerance of uncertainty, and self-compassion).