Objectives: Shared decision-making (SDM) in health care is widely encouraged. However, for SDM to occur patients need to be able to obtain, understand and apply medical information. Our aim was to assess the relationship between health literacy (HL), numeracy and SDM (using French translations of validated measures). Methods: A cross-sectional survey using a self-administered online questionnaire was proposed to all members of the Seintinelles association. Several scales were used to measure HL (FCCHL and 3HLQ/SILS), numeracy (SNS-3), the SDM process (CollaboRATE) and explore their interrelationships. Results: Data from 2 299 respondents (96.7% women, 46.1% with a history of cancer) were analysed. All measurement scales showed adequate psychometric properties. Functional HL, communicative HL and numeracy were positively associated with SDM while no significant relation was observed between critical HL and SDM. Furthermore, perceived difficulties in asking physicians' questions and deprivation were negatively associated with SDM. Conclusion: Patient support to reach SDM requires high levels of HL, particularly in the functional and communicative domains. Efforts must be made to improve access and understanding of health information. Practice implications: Brief self-reported measures could be used to screen for low levels of health literacy, tailor information accordingly and improve patient involvement in healthcare decision-making.