BACKGROUND: Funders and scientific journals use peer review to decide which projects to fund or articles to publish. Reviewer training is an intervention to improve the quality of peer review. However, studies on the effects of such training yield inconsistent results, and there are no up-to-date systematic reviews addressing this question. OBJECTIVES: To evaluate the effect of peer reviewer training on the quality of grant and journal peer review. SEARCH METHODS: We used standard, extensive Cochrane search methods. The latest search date was 27 April 2022. SELECTION CRITERIA: We included randomized controlled trials (RCTs; including cluster-RCTs) that evaluated peer review with training interventions versus usual processes, no training interventions, or other interventions to improve the quality of peer review. DATA COLLECTION AND ANALYSIS: We used standard Cochrane methods. Our primary outcomes were 1. completeness of reporting and 2. peer review detection of errors. Our secondary outcomes were 1. bibliometric scores, 2. stakeholders' assessment of peer review quality, 3. inter-reviewer agreement, 4. process-centred outcomes, 5. peer reviewer satisfaction, and 6. completion rate and speed of funded projects. We used the first version of the Cochrane risk of bias tool to assess the risk of bias, and we used GRADE to assess the certainty of evidence. MAIN RESULTS: We included 10 RCTs with a total of 1213 units of analysis. The unit of analysis was the individual reviewer in seven studies (722 reviewers in total), and the reviewed manuscript in three studies (491 manuscripts in total). In eight RCTs, participants were journal peer reviewers. In two studies, the participants were grant peer reviewers. The training interventions can be broadly divided into dialogue-based interventions (interactive workshop, face-to-face training, mentoring) and one-way communication (written information, video course, checklist, written feedback). Most studies were small. We found moderate-certainty evidence that emails reminding peer reviewers to check items of reporting checklists, compared with standard journal practice, have little or no effect on the completeness of reporting, measured as the proportion of items (from 0.00 to 1.00) that were adequately reported (mean difference (MD) 0.02, 95% confidence interval (CI) -0.02 to 0.06; 2 RCTs, 421 manuscripts). There was low-certainty evidence that reviewer training, compared with standard journal practice, slightly improves peer reviewer ability to detect errors (MD 0.55, 95% CI 0.20 to 0.90; 1 RCT, 418 reviewers). We found low-certainty evidence that reviewer training, compared with standard journal practice, has little or no effect on stakeholders' assessment of review quality in journal peer review (standardized mean difference (SMD) 0.13 standard deviations (SDs), 95% CI -0.07 to 0.33; 1 RCT, 418 reviewers), or change in stakeholders' assessment of review quality in journal peer review (SMD -0.15 SDs, 95% CI -0.39 to 0.10; 5 RCTs, 258 reviewers). We found very low-certainty evidence that a video course, compared with no video course, has little or no effect on inter-reviewer agreement in grant peer review (MD 0.14 points, 95% CI -0.07 to 0.35; 1 RCT, 75 reviewers). There was low-certainty evidence that structured individual feedback on scoring, compared with general information on scoring, has little or no effect on the change in inter-reviewer agreement in grant peer review (MD 0.18 points, 95% CI -0.14 to 0.50; 1 RCT, 41 reviewers, low-certainty evidence). AUTHORS' CONCLUSIONS: Evidence from 10 RCTs suggests that training peer reviewers may lead to little or no improvement in the quality of peer review. There is a need for studies with more participants and a broader spectrum of valid and reliable outcome measures. Studies evaluating stakeholders' assessments of the quality of peer review should ensure that these instruments have sufficient levels of validity and reliability.