Background: Depression is a worldwide problem warranting global solutions to tackle it. Enhancing well-being has benefits in its own right and could be a good strategy for preventing depression. Providing well-being interventions via the Internet may have synergetic effects. Objective: Psyfit ("mental fitness online") is a fully automated self-help intervention to improve well-being based on positive psychology. This study examines the clinical effects of this intervention. Methods: We conducted a 2-armed randomized controlled trial that compared the effects of access to Psyfit for 2 months (n=143) to a waiting-list control condition (n=141). Mild to moderately depressed adults in the general population seeking self-help were recruited. Primary outcome was well-being measured by Mental Health Continuum-Short Form (MHC-SF) and WHO Well-being Index (WHO-5); secondary outcomes were depressive symptoms, anxiety, vitality, and general health measured by Center for Epidemiological Studies Depression Scale (CES-D), Hospital Anxiety and Depression Scale Anxiety subscale (HADS-A), and Medical Outcomes Study-Short Form (MOS-SF) vitality and general health subscales, respectively. Online measurements were taken at baseline, 2 months, and 6 months after baseline. Results: The dropout rate was 37.8% in the Psyfit group and 22.7% in the control group. At 2-month follow-up, Psyfit tended to be more effective in enhancing well-being (nonsignificantly for MHC-SF: Cohen's d=0.27, P=.06; significantly for WHO-5: Cohen's d=0.31, P=.01), compared to the waiting-list control group. For the secondary outcomes, small but significant effects were found for general health (Cohen's d=0.14, P=.01), vitality (d=0.22, P=.02), anxiety symptoms (Cohen's d=0.32, P=.001), and depressive symptoms (Cohen's d=0.36, P=.02). At 6-month follow-up, there were no significant effects on well-being (MHC-SF: Cohen's d=0.01, P=.90; WHO-5: Cohen's d=0.26, P=.11), whereas depressive symptoms (Cohen's d=0.35, P=.02) and anxiety symptoms (Cohen's d=0.35, P=.001) were still significantly reduced compared to the control group. There was no clear dose-response relationship between adherence and effectiveness, although some significant differences appeared across most outcomes in favor of those completing at least 1 lesson in the intervention. Conclusions: This study shows that an online well-being intervention can effectively enhance well-being (at least in the short-term and for 1 well-being measure) and can help to reduce anxiety and depression symptoms. Further research should focus on increasing adherence and motivation, reaching and serving lower-educated people, and widening the target group to include people with different levels of depressive symptoms.