ABSTRACT Aim To co‐design a nurse‐led family support intervention for patients with cancer and their family members. Design An intervention co‐design process. Methods The Experience‐Based Co‐Design method was conducted in a hospital in northern Spain from January 2024 to February 2025 with nurses, patients and family members, following The Point of Care Foundation's eight steps: observations, individual interviews, editing the trigger film, feedback events, co‐design and validation events and celebration event. Results Four themes emerged from the integrated dataset. (1) Cancer: a family affair, (2) Needs of patients with cancer and their families, (3) The importance of establishing a trust‐based nurse‐family relationship and (4) Barriers and facilitators for a family‐oriented approach in care. These findings informed feedback events where stakeholders identified critical intervention areas, including family support, illness communication, emotional expression and the importance of trust. Based on these results, the Family CARE‐ON intervention was structured around three core components: Engaging, Supporting and Empowering. Conclusions This study highlights the value of the Experience‐Based Co‐Design methodology, with active engagement from stakeholders, to develop a family support intervention in oncology care. Further research is needed to evaluate the feasibility and effectiveness, as well as to explore the implementation and scalability of the intervention in oncological settings. Implications for the Profession and/or Patient Care Co‐designing a family support intervention together with stakeholders ensures alignment with their needs and expectations, while also fostering the feasibility of the intervention in clinical practice. Impact The results show how stakeholders co‐designed a family support intervention, drawing from their own experiences and perspectives on the impact of cancer on the family. Reporting Method Guidance for reporting intervention development studies in health research (GUIDED) and Template for Intervention Description and Replication (TIDieR). Patient or Public Contribution Nurses, patients and family members were involved in designing the family support intervention.