Greater optimism is related to better mental and physical health. A number of studies have investigated interventions intended to increase optimism. The aim of this meta-analysis was to consolidate effect sizes found in randomized controlled intervention studies of optimism training and to identify factors that may influence the effect of interventions. Twenty-nine studies, with a total of 3319 participants, met criteria for inclusion in the analysis. A significant meta-analytic effect size, g  =  .41, indicated that, across studies, interventions increased optimism. Moderator analyses showed that studies had significantly higher effect sizes if they used the Best Possible Self intervention, provided the intervention in person, used an active control, used separate positive and negative expectancy measures rather than a version of the LOT-R, had a final assessment within one day of the end of the intervention, and used completer analyses rather than intention-to-treat analyses. The results indicate that psychological interventions can increase optimism and that various factors may influence effect size.