Reducing smoking rates among socially disadvantaged populations is a key component of reducing social inequalities in health. To achieve this, in addition to preventing smoking initiation among these populations, it is important to develop effective smoking cessation interventions tailored to this objective. The StopAdvisor project, led by a British team, is an online smoking cessation support tool. This article provides a summary of a body of work dedicated to the design and evaluation of StopAdvisor. The website’s design was based on Robert West’s PRIME (Plans, Responses, Impulses, Motives, Evaluations) theory of motivation; it was tested among socially disadvantaged British smokers (those with low educational attainment, unemployed, or manual laborers). A randomized controlled trial was conducted with 4,613 smokers, half of whom were socially disadvantaged. The primary outcome measure was 6 months of continuous abstinence verified through biological testing. The study demonstrated that the interactive StopAdvisor tool was more effective than a simple static informational website among socially disadvantaged smokers, whereas it was not more effective for advantaged smokers. For smokers who do not wish to engage in face-to-face psychological counseling, an online smoking cessation tool could therefore be an effective alternative, particularly for smokers with a low socioeconomic status.