Background: The alliance between therapists and patients is one of the most investigated factors of psychotherapy and a robust predictor of treatment outcome in face-to-face interventions. The implementation of internet-based interventions for psychological disorders has become more and more relevant over the past years, particularly in European countries. Regarding the relevance of the alliance for therapy outcome in internet-based interventions, mixed results are reported. The present correlative meta-analysis systematically summarizes the relation between alliance and therapy outcome in internet-based interventions for psychological disorders. Methods: Based on a systematic search, we detected 18 internet-based studies with 23 independent samples (1,177 participants). Results: The omnibus revealed a moderate effect of r = 0.252, p < 0.001, 95% CI [0.19, 0.31]. The analyses of the subscales indicated that the task component (r = 0.28) was slightly more predictive than the bond component (rbond = 0.16; QM(3) = 26.85, p < 0.001). None of the investigated moderators indicated significance. Discussion: This meta-analysis underlines the assumption that the alliance is a robust facilitative factor across mental health treatments.