The last decades experienced a rapid growth in the number of studies examining the effects of psychological interventions on well-being, yet well-being is often conceptualized and measured in different ways in these studies. Previous meta-analyses included studies with a plethora of different well-being instruments, which provides an ambiguous picture of the effectiveness. Furthermore, prior meta-analyses mainly included specific types of psychological interventions. The goal of the current study was to synthesize the effectiveness of psychological interventions in improving well-being as measured with one consistent and comprehensive well-being instrument, the Mental Health Continuum (MHC). The literature was searched for RCTs examining the effect of psychological interventions in both clinical and non-clinical populations that used the MHC as outcome. 46 RCTs (N = 7,618) and 64 comparisons were analyzed using 3-level meta-analysis models. When compared with non-active control groups, small significant effects were found for total well-being at posttest (beta = 0.25), and for the subscales emotional (beta = 0.27), social (beta = 0.25), and psychological well-being (beta = 0.30). Effects were smaller but still significant at follow-up. Subgroup analyses yielded significantly stronger effects for guided compared with non-guided interventions and for studies with good quality. Effects were similar for clinical and non-clinical populations and specific types of interventions. Mindfulness and ACT interventions significantly improved well-being. These findings suggest that psychological interventions can improve well-being, and that different interventions have the potential to improve well-being. Effects also seem to be independent of other factors, including delivery mode, format or target group.