Objective: The aim of this study was to examine the efficacy and effectiveness of a theory-driven multicomponent positive psychology intervention to improve well-being for individuals with severe psychiatric conditions (SPCs) in comparison with treatment as usual (TAU). This intervention moves away from the traditional psychiatric perspectives that focused on symptoms and deficits, promoting a broader view of outcomes such as noncritical self-acceptance, strengths, and positive relationships with others, among other things. Method: A two-arm randomized and outcome-blinded trial with pre-post and 6-month follow-up evaluations was conducted to assess the intervention. A total of 141 participants were allocated to either the experimental condition or the waiting list group receiving their TAU. A mixed-effect model was used to examine the efficacy of the intervention and a repeated-measures Student’s t-test for the follow-up effectiveness analysis. Results: The intervention protocol was highly acceptable for participants, showing very high participant satisfaction as well as good attendance and adherence rates. At the end of the group therapy, participants reported a significant improvement in self-acceptance and environmental mastery and, these changes were maintained in the follow-up assessment after 6 months. Conclusions and Implications for Practice: The results of this randomized control trial provide further evidence supporting that positive psychology approaches can be a powerful complementary strategy to promote more comprehensive psychiatric rehabilitation services for people with SPCs. (PsycInfo Database Record (c) 2022 APA, all rights reserved)