Background Smoking remains a leading cause of illness and preventable death. NHS Stop Smoking Services increase quitting, but, as access is in decline, cost-effective interventions are needed that promote these services. StopApp™ (Coventry University, Coventry, UK) is designed to increase booking and attendance at Stop Smoking Services. Design A two-arm feasibility randomised controlled trial of StopApp (intervention) compared with standard promotion and referral to Stop Smoking Services (control) was conducted to assess recruitment, attrition and health equity of the design, alongside health economic and qualitative process evaluations. Setting Smokers recruited via general practitioners, community settings and social media. Participants Smokers aged ≥ 16 years were recruited in one local authority. Participants had to live or work within the local authority area, and there was a recruitment target of 120 participants. Interventions StopApp to increase booking and attendance at Stop Smoking Services. Main outcome measures Participants completed baseline measures and follow-up at 2 months post randomisation entirely online. Objective data on the use of Stop Smoking Services were collected from participating Stop Smoking Services, and age groups, sex, ethnicity and socioeconomic status in baseline recruits and follow-up completers/non-completers were assessed for equity. Results Eligible participants (n = 123) were recruited over 116 days, with good representation of lower socioeconomic status groups; black, Asian and minority ethnic groups; and all age groups. Demographic profiles of follow-up completers and non-completers were broadly similar. The attrition rate was 51.2%, with loss to follow-up lowest in the social media setting (n = 24/61; 39.3%) and highest in the general practitioner setting (n = 21/26; 80.8%). Most measures had < 5% missing data. Social media represented the most effective and cost-efficient recruitment method. In a future, definitive, multisite trial with recruitment driven by social media, our data suggest that recruiting ≥ 1500 smokers over 12 months is feasible. Service data showed that five bookings for the Stop Smoking Services were scheduled using StopApp, of which two did not attend. Challenges with data access were identified. A further five participants in the intervention arm self-reported booking and accessing Stop Smoking Services outside StopApp compared with two control arm participants. Event rate calculations for the intervention were 8% (Stop Smoking Services data), 17% (including self-reports) and 3.5% from control arm self-reports. A conservative effect size of 6% is estimated for a definitive full trial. A sample size of 840 participants would be required to detect an effect for the primary outcome measure of booking a Stop Smoking Services appointment in a full randomised controlled trial. The process evaluation found that participants were satisfied with the research team contact, study methods and provision of e-vouchers. Staff interviews revealed positive and negative experiences of the trial and suggestions for improvements, including encouraging smokers to take part. Conclusion This feasibility randomised controlled trial found that, with recruitment driven wholly or mainly by social media, it is possible to recruit and retain sufficient smokers to assess the effectiveness and cost-effectiveness of StopApp. The study methods and measures were found to be acceptable and equitable, but accessing Stop Smoking Services data about booking, attendance and quit dates was a challenge. A full trial may be feasible if service data are accessible. This will require careful planning with data controllers and a targeted social media campaign for recruitment. Changes to some study measures are needed to avoid missing data, including implementation of a more intensive follow-up data collection process. Future work We plan a full, definitive randomised controlled trial if the concerns around data access can be resolved, with adaptations to the recruitment and retention strategy. Limitations Our trial had high attrition and problems with collecting Stop Smoking Services data, which resulted in a reliance on self-reporting. Trial registration Research Registry: 3995. The trial was registered on 18 April 2018. Funding This project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full in Public Health Research; Vol. 9, No. 5. See the NIHR Journals Library website for further project information.