BACKGROUND: Although support for tobacco smoking cessation provided by general practitioners (GP) has proven effective, such an approach remains quite rare. We designed a behavioural science-based intervention to improve GP support for patients to quit smoking. The present study aimed to evaluate the effectiveness of this in-office paper-based intervention by comparing it with a no-intervention control group. METHODS: We conducted an online randomised controlled trial between January and May 2024 among GPs in France. The intervention consisted in sending a kit containing an information sheet for GPs outlining the main stages of brief advice for smoking cessation, questionnaires for patients asking them about their smoking status and, for those who smoked, their motivation to quit, as well as a poster to be displayed in each GP's waiting room to encourage people who smoke tobacco to complete the questionnaire. The primary outcome was the proportion of people who smoke with whom GPs had discussed smoking cessation on their last full working day five weeks after the kit had been sent out. The secondary outcome was the likelihood that GPs provided cessation support (e.g., prescribed nicotine replacement therapy (NRT), proposed a follow-up consultation, and/or referred patients to other professionals) on the same day. RESULTS: Of the 800 GPs recruited, 641 fully completed the final survey (333 in the intervention group and 308 in the control group). The proportion of people who smoke with whom GPs had discussed cessation was significantly higher in the intervention group (59.0% vs. 52.3%, p < 0.05). The likelihood of a follow-up consultation being offered to patients who smoke was also significantly higher in the intervention group (73.8% vs. 60.3%, p < 0.05). The likelihood of NRT prescription or referral were not significantly different between groups (82.4% vs. 77.9%, and 23.3% vs. 21.5%, respectively). CONCLUSIONS: Providing simple paper-based tools was associated with a greater likelihood of GPs discussing smoking cessation with their patients. This kind of intervention could usefully complement other primary care interventions, for example training, remuneration and financial incentives or care pathway reorganisation, with a view to significantly decreasing smoking prevalence. TRIAL REGISTRATION: The study was retrospectively registered on 25 November 2024 (ISRCTN10207960).