OBJECTIVE: Mindfulness interventions (MIs) train nonjudgmental attention to present-moment experience and aim to improve mental health and well-being. The evidence for their effect on interpersonal relationships is promising but uncertain. This study examines the effect of MIs on couple relationship satisfaction (RS). METHOD: Randomized controlled trials of MIs including RS were selected based on systematic searches in Web of Science, PubMed, APA PsycInfo, Embase, Cochrane Central, ProQuest, and Google Scholar. We applied three-level meta-analysis with robust variance estimation to pool effects and multimodel approaches to explore moderators. RESULTS: We calculated 90 effect sizes (k) nested within 28 studies (K) including 6,097 participants in a couple relationship. MIs had a significant medium effect on RS with high heterogeneity (g = 0.60, 95% confidence interval [0.16, 1.04], I2 = 97 [95, 99]). The effect on RS was influenced by extreme outliers (e.g., g up to 7.48). Removing outliers resulted in a significant small effect with low heterogeneity (g = 0.21 [0.11, 0.31], I2 = 25 [0, 67], k = 85, K = 26). Effects were moderated by intervention length, baseline satisfaction, and risk of bias. There were significant effects for both clinical and community samples. The certainty of the evidence is very low due to inconsistency, imprecision, risk of bias, and suspicion of publication bias. Generalization is limited by insufficient reporting. CONCLUSIONS: This meta-analysis indicates that MIs have a consistent small effect on RS, but the quality of evidence points to the need for program theory and rigorous methodology. (PsycInfo Database Record (c) 2025 APA, all rights reserved).