BACKGROUND AND AIMS: Smartphone applications (apps) hold promise for delivering tobacco smoking cessation support to large numbers of people at low unit cost. Smoke Free is an evidence-informed, widely used app that is highly rated by users. This study aims to assess its effectiveness compared with no support. DESIGN: Two-arm individually randomized controlled effectiveness trial. SETTING: Online trial with no restrictions on location. PARTICIPANTS: English-speaking smokers aged ≥ 18 years willing to make a quit attempt within 4 weeks from initial contact (n = 4990). MEASUREMENTS: The primary outcome measure is self-reported 26-week continuous smoking abstinence. Secondary outcome measures include quit attempts in the first 4 weeks post-randomization, 12-week continuous smoking abstinence and 26-week continuous smoking abstinence among those who made a quit attempt. COMMENTS: If it is effective, the Smoke Free smartphone app is an affordable and widely implementable intervention to help smokers to quit.