Abstract AimTo describe the co-designing process of an online support programme with and for informal carers of people with heart failure.DesignA co-design process built on core concepts and ideas embedded in co-design methodology.Data sources Our co-design process included three phases involving 32 informal caregivers and 25 content creators; (1) Identification of topics and content through literature searches, focus group interviews and user group sessions; (2) Development of the online support programme and; (3) Refinement and finalization which included testing a paper prototype followed by testing the online version and testing and approval of the final version of the support programme.Outcomes The co-design process resulted in a support programme consisting of 15 different modules relevant to informal carers, delivered on a National Health Portal.Conclusion Co-design is an explorative process where researchers need to balance a range of potentially conflicting factors and to ensure that the end users are genuinely included in the process.Relevance to clinical practice Emphasizing equal involvement of end users (e.g. carers or patients) in the design and development of healthcare interventions aligns with contemporary ideas of person-centred care and provides a valuable learning opportunity for those involved. Furthermore, a co-designed online support programme has the capacity to be both accessible and meet end users' information and support needs, thereby optimizing their self-care abilities. Additionally, an online support programme provides the opportunity to address current challenges regarding scarce resources and the lack of healthcare personnel.Reporting methodsConsolidated criteria for reporting qualitative research (COREQ).Patient or public contributionBoth informal carers and content creators were involved in developing the support programme.