Individuals who are higher in subjective well-being not only feel happier, they are more likely have fulfilling relationships, increased work performance and income, better physical health, and longer lives. Over the past several decades, the science of subjective well-being has produced insights into these benefits of happiness, and—recognizing their importance—has begun to examine the factors that lead to greater well-being, from cultivating strong relationships to pursuing meaningful goals. However, studies to date have typically focused on improving subjective well-being by intervening with singular constructs, using primarily college student populations, and were short-term in nature. Moreover, little is understood about the impact of a well-being treatment delivered online vs. in-person. In the present article, we describe a comprehensive intervention program including 3-month initial treatment followed by a 3-month follow-up, ENHANCE: Enduring Happiness and Continued Self-Enhancement. One-hundred and sixty participants will be recruited from two different sites to participate in one of two versions of ENHANCE: in-person (n = 30) vs. wait-list control (n = 30); or online (n = 50) vs. wait-list control (n = 50). Assessments will be completed at baseline, three months and six months. Our primary outcome is change in subjective well-being across treatment (3 months) and follow-up (6 months). Secondary outcomes include self-report and objective measures of health, as well as a psychological mediators (e.g., psychological needs) and moderators (e.g., personality) of treatment outcomes. We hope to provide researchers, practitioners, and individuals with an evidence-based treatment to improve happiness and subjective well-being.