AIMS: There is an increasing awareness of the evidence-based selection of outcomes to be measured in clinical trials and clinical practice. Currently, there is no core outcome set (COS) for cardio-oncology, which may hinder the (inter)national comparison of the effectiveness of research and the quality of cardio-oncology care. The aim of this study is to develop a standard and pragmatic patient-centred outcome set to assess and monitor cancer patients and survivors at risk of or with cardiovascular diseases. METHODS AND RESULTS: A list of outcome domains was generated through a review of registries and guidelines, and six patient interviews. The project team reviewed and refined the outcome domains prior to starting a two-round Delphi procedure conducted between January and June 2022. The panellists, including healthcare providers and researchers, were invited to rate the importance of the outcomes. Twenty-six experts from 11 countries rated a list of 93 outcomes (round 1) and 63 outcomes (round 2) to gain consensus on a list of outcome measures, and of demographic factors, health status, and treatment variables. The final COS includes 15 outcome measures, reflecting four core areas: life impact (n = 2), pathophysiological manifestations (n = 9), resource use/economic impact (n = 1), and mortality/survival (n = 3). Next, 6 demographic factors, 21 health status, 3 cardiovascular, and 9 cancer variables were included. CONCLUSIONS: This is the first international development of a COS for cardio-oncology. This set aims to facilitate (inter)national comparison in cardio-oncology care, using standardized parameters and meaningful patient-centred outcomes for research and quality of care assessments.