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.