Integration of exercise into standard oncology care requires a highly skilled workforce of exercise professionals; however, competency requirements have not kept pace with advancements in the field. Therefore, the aim of this study was to obtain consensus on core competencies required for an exercise professional to be qualified to work with adults undergoing active cancer treatment. A three-round modified electronic Delphi process was used. In Round 1, an international group of 64 exercise oncology stakeholders (i.e., exercise oncology professionals (n = 29), clinical referrers (n = 21), and people with lived experience (n = 14)) responded to open-ended prompts eliciting perspectives regarding competencies needed for an exercise oncology professional to work with adults receiving active cancer treatment. Subsequently, only exercise oncology professionals participated, ranking the importance of competencies. In Round 2, professionals received summary feedback, ranked new competencies generated from open-ended responses, and reranked competencies not reaching consensus. In the final round, professionals finalized consensus ranking and rated frequency and mastery level for each. Consensus was reached on 103 core competencies required for exercise professionals to be qualified to deliver care to adults undergoing active cancer treatment. The core competencies represent 10 content areas and reflect the needs of clinical referrers and people with lived experience of receiving cancer treatment. The core competencies identified reflect significant advancements in the field of exercise oncology. Results will underpin the development of education, certification, and employment requirements for exercise oncology professionals, providing a critical step toward achieving routine integration of exercise into standard oncology care.