Clinical perfectionism has been found to be a risk and maintaining factor in eating disorders (EDs), compulsive exercise, and athlete burnout. This study investigated whether an unguided internet cognitive-behavioral treatment (ICBT) for perfectionism would reduce ED pathology, compulsive exercise, and burnout in individuals who engage in regular exercise. Participants were randomly allocated to intervention (n = 38) or waitlist control (n =29). A generalized linear mixed model (GLMM) analysis was conducted pre and post treatment. A follow-up analysis was conducted with the intervention group at 3 and 6 months. The intervention group experienced a significant reduction in perfectionism (FMPS-CM: F[1,117] = 17.53, p = < .001, Cohen's d = .82), ED symptomology (EDE-Q: F[1,55] = 7.27, p = .009,Cohen's d = .53) and compulsive exercise (CET: F[1,116] = 10.33, p < .001,Cohen's d = .63). The changes attained post-treatment were maintained within the intervention group at 3-month (FMPS-CM (t[1,100] = 3.67, p < . 001, Cohen's d = .85) (EDE-Q (t[1,50] = 2.20, p = .03, Cohen's d = 1.26) and 6-month follow-up (FMPS (t[1,100] = 2.74, p = 007, Cohen's d = .70) (EDE-Q (t[1,50] = 2.18, p = .03, Cohen's d = 1.26). The results indicate unguided ICBT for perfectionism can have a significant impact on perfectionism, compulsive exercise, and ED symptomatology.