Aims To estimate the short‐term effectiveness, feasibility and acceptability of a smoking cessation intervention (the iQuit system) that consists of tailored printed and Short Message Service (SMS) text message self‐help delivered as an adjunct to cessation support in primary care to inform the design of a definitive trial. Design A stratified two parallel‐group randomized controlled trial comparing usual care (control) with usual care plus the iQuit system (intervention), delivered by primary care nurses/healthcare assistants who were blinded to the allocation sequence. Setting Thirty‐two general practice (GP) surgeries in England, UK. Participants A total of 602 smokers initiating smoking cessation support from their local GP surgery were randomized (control n = 303, intervention n = 299). Measurements Primary outcome was self‐reported 2‐week point prevalence abstinence at 8 weeks follow‐up. Secondary smoking outcomes and feasibility and acceptability measures were collected at 4 weeks after quit date, 8 weeks and 6 months follow‐up. Findings There were no significant between‐group differences in the primary outcome [control 40.3%, iQuit 45.2%; odds ratio (OR) = 1.22, 95% confidence interval (CI) = 0.88–1.69] or in secondary short‐term smoking outcomes. Six‐month prolonged abstinence was significantly higher in the iQuit arm (control 8.9%, iQuit 15.1%; OR = 1.81, 95% CI = 1.09–3.01). iQuit support took on average 7.7 minutes (standard deviation = 4.0) to deliver and 18.9% (95% CI = 14.8–23.7%) of intervention participants discontinued the text message support during the programme. Conclusions Tailored printed and text message self‐help delivered alongside routine smoking cessation support in primary care does not significantly increase short‐term abstinence, but may increase long‐term abstinence and demonstrated feasibility and acceptability compared with routine cessation support alone.