Objectives <p>This systematic review examined previous findings on the effects of mindfulness-based interventions (MBIs) on pre-service and in-service K-12 teachers’ general mindfulness, mindful teaching (i.e., intra- and interpersonal mindfulness), and self-efficacy. In addition, we used the treatment fidelity tool for MBIs to descriptively examine the associations between implementation fidelity and findings from the identified MBIs. Moreover, we examined the quality of the included studies.</p> Method <p>Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines, 21 experimental and quasi-experimental studies from three databases were determined eligible for inclusion in this review.</p> Results <p>Findings from these studies demonstrated that MBIs generally have significant positive effects on general mindfulness, intrapersonal mindfulness, and self-efficacy. Additionally, the small set of studies that included interpersonal mindfulness showed no significant effects on this construct. Furthermore, no associations were found between the total number of reported implementation fidelity domains and impacts on teacher outcomes, and 19 studies were rated as “good” in the quality appraisal process.</p> Conclusions <p>MBIs are an effective approach for improving teachers’ general mindfulness, intrapersonal mindfulness, and self-efficacy. In addition, this review provided preliminary empirical support for further investigating the effects of MBIs on teachers’ self-efficacy through general and intrapersonal mindfulness. MBIs could benefit from incorporating practices that not only address intrapersonal but also interpersonal mindfulness. Future research should address the lack of statistical power highlighted in the included studies with null findings and investigate more systematically how variation in implementation fidelity could impact the effects of MBIs on teacher outcomes.</p> Preregistration <p>This study was not preregistered. </p>