This study provides a quantitative synthesis of meta-analytic evidence for the effectiveness of very broadly defined positive psychological interventions (PPIs), i.e. interventions that enhance well-being through pathways consistent with positive psychology theory. The definition covers a wide range of PPIs including single and multi-element PPI programs as well as mindfulness, mind-body, and physical exercise-based interventions. Five databases were searched. One hundred and ninety eight meta-analyses involving 4,065 primary studies and 501,335 participants were selected for review. Using the AMSTAR-2 criteria, the selected meta-analyses were found to be of moderate or high quality. At post-intervention, PPIs had a significantly small to medium effect on well-being, QoL, strengths, depression, anxiety, and stress. Gains were partially maintained at 7.5-month follow-up. Mind-body PPIs such as yoga were particularly effective. Individuals who engaged in longer, face-to-face programs benefited most. This mega-analysis shows that PPIs have an extensive evidence base supporting their effectiveness.