Background: Each year, more than a quarter of smokers try to quit, most without outside help. Support from a general practitioner (GP) for smoking cessation is effective, but remains too infrequent. A behavioral science-based intervention was designed to encourage GPs to address smoking cessation. The objective of the study was to evaluate the effectiveness of this intervention. Method: An online randomized controlled trial was conducted between January and May 2024 among GPs in mainland France practicing in private practice. The intervention consisted of sending a kit containing an advisory sheet for physicians outlining the main steps of brief intervention, questionnaires for patients asking about their smoking status and motivation to quit, and a poster to be displayed in the waiting room to encourage smokers to complete the questionnaire. Participants in the control group received no intervention. The primary efficacy outcome was the proportion of smoking patients with whom the topic of smoking cessation had been discussed on the last workday of the surveyed GPs, one month after the tools were sent. The secondary efficacy outcome was the probability that support (prescription of nicotine replacement therapy (NRT), offer of a follow-up consultation, referral to other professionals) was offered on the last day of work. Qualitative interviews were also conducted with 12 GPs who received the intervention. Results: 800 GPs were recruited, and 641 of them responded to the follow-up survey (333 in the intervention group and 308 in the control group). The proportion of smoking patients with whom smoking cessation had been discussed was higher in the intervention group (59.0%) than in the control group (52.3%, p