Positive psychology is paving the way for interventions that enduringly enhance well-being and the internet offers the potential to disseminate these interventions to a broad audience in an accessible and sustainable manner. There is now sufficient evidence demonstrating the efficacy of internet interventions for mental illness treatment and prevention, but little is known about enhancing well-being. The current study examined the efficacy of a positive psychology internet-based intervention by adopting a randomised controlled trial design to compare a strengths intervention, a problem solving intervention and a placebo control. Participants (n = 160) completed measures of well-being (PWI-A, SWLS, PANAS, OTH) and mental illness (DASS-21) at pre-assessment, post-assessment and 3-month follow-up. Well-being increased for the strengths group at post- and follow-up assessment on the PWI-A, but not the SWLS or PANAS. Significant changes were detected on the OTH subscales of engagement and pleasure. No changes in mental illness were detected by group or time. Attrition from the study was 83% at 3-month follow-up, with significant group differences in adherence to the intervention: strengths (34%), problem solving (15.5%) and placebo control (42.6%). Although the results are mixed, it appears possible to enhance the cognitive component of well-being via a self-guided internet intervention.