Stress contributes to the development and maintenance of obesity. Mindfulness-based therapies are being used to reduce stress and promote weight reduction and maintenance. This study aimed to determine the efficacy of mindfulness-based interventions for stress and weight reduction in the short, medium, and long term. Searches on PsycINFO, Medline, CINAHL, Scopus, WOS, and Science Direct were conducted until March 2021. Intervention studies with a sample of adults were included; these evaluated a mindfulness-based intervention and used stress and weight or body mass index as outcome variables. These criteria were met by 13 articles. A meta-analysis of 8 of the 13 articles was performed with a random-effects or fixed-effects model, depending on the level of heterogeneity between studies. Mindfulness-based interventions had a small effect on stress reduction over a 3-month period: effect size (standardized mean difference) = -0.29 (95% CI: -0.49, -0.10). However, no significant evidence was found for stress reduction from 3 months onwards, nor for weight or body mass index reduction in any period. Mindfulness-based interventions are effective in reducing stress in the short term, but not in the medium or long term, nor are they effective for weight or body mass index. More robust and longer study designs are needed to determine their effects.