The Partners for Change Outcome System (PCOMS) is a feedback system that has been developed as part of psychotherapeutic treatment. The aim of this systematic review was to evaluate the effect of the PCOMS. We searched the literature and included studies that used a randomized controlled trial (RCT) design. We calculated a combined effect size across studies for outcomes related to the number of sessions attended. We also calculated a combined effect size for outcomes related to the participants' well-being. However, in the analysis of the effect on well-being, we excluded studies that included only the Outcome Rating Scale as a measure of effect because this scale is part of the PCOMS. In the calculation of the combined effect size, we used random effect models with inverse weighted variance. In the systematic literature search we identified 14 RCT studies that evaluated the effect of the PCOMS. Based on 12 studies, we found a rather small effect size for the number of sessions attended favoring the PCOMS intervention (Hedges's g = 0.13; 95% confidence interval [CI: 0.001, 0.26]). The effect size corresponded to a difference of less than 1 session. Six studies included a well-being scale that was independent of the PCOMS intervention as the outcome. The effect size for the 6 studies was insignificant (Hedges's g = 0.03; 95% Cl [-0.18, 0.23]). We found no evidence that the PCOMS feedback system has an effect on the number of sessions attended by clients or that the PCOMS improves the well-being of clients. (PsycInfo Database Record (c) 2020 APA, all rights reserved).