Abstract Objective: To provide a comprehensive assessment of the association between psychological treatment adherence/competence/integrity (ACI) and clinical outcomes. Method: The review protocol was preregistered (CRD42020193889). Studies that assessed ACI-outcome relationships for adult psychotherapy were searched across three databases (Scopus, PsycINFO, MEDLINE). Random effects meta-analyses were conducted on adherence-outcome, competence-outcome, and integrity-outcome relationships. Separate analyses were performed for studies with hierarchical (i.e., patients nested within therapist) versus nonhierarchical study designs. Moderator analyses were performed according to predefined clinical and methodological features. GRADE assessments rated the quality of each meta-analytic comparison. Results: The review identified 62 studies suitable for inclusion (45 adherence-outcome, 39 competence-outcome, and seven integrity-outcome effect sizes; N = 8,210 across all analyses). No significant adherence-outcome association was found. A small significant positive association was found only in nonhierarchical studies between competence and outcome, r = 0.17, 95% CI [0.07-0.26], p < .001, ∼d = .34, GRADE = moderate. Small-to-moderate significant positive associations between integrity and outcome were found for both nonhierarchical, r = 0.15, 95% CI [0.06-0.23], p < .001, ∼d = .30, GRADE = high, and hierarchical study designs, r = 0.23, 95% CI [0.01, 0.43], p < .044, ∼d = .47, GRADE = low. Diagnosis, treatment modality and year of publication significantly moderated the strength of ACI-outcome correlations. Conclusions: Competence and integrity are significantly associated with clinical outcome, with a magnitude comparable to wider common factors. Further research is required to study these process-outcome associations with greater precision in routine-care settings and to understand the role of moderating variables. (PsycInfo Database Record (c) 2022 APA, all rights reserved).