Objectives: Existing research on the perspectives of patients with cancer and health care professionals indicates that patient decision making on cancer clinical trial participation is a complex process and may be poorly understood, possibly compromising their decision to participate. This systematic review investigates interventions that support patients in their decision-making processes regarding whether to participate or not and assesses the qualities of the interventions, measures used and related outcomes. Methods: Six databases were systematically searched and only studies evaluating interventions that support the decision making of adult patients offered to enter a cancer clinical trial were included. Ten articles met the criteria and were analysed using a narrative synthesis approach. Results: The research focus of the included studies reflected the multifactorial nature of what constitutes support for patient decision making in terms of entering a cancer clinical trial. However, most interventions were based on the hypothesis that more information leads to support in decision making, and did not take other factors, such as the relationship to the clinical staff or relatives, the patients’ strong hope for therapeutic benefit or other existential needs into account. The interventions were primarily based on a specific tool, executed once, which seems to imply that decisions need only to be supported once and not at several time points throughout the decision process, and did not assess the importance of a patient’s family- or social relations. Moreover, few interventions focused on the patients’ counselling experience or assessed patient preferences in relation to decision making. Conclusions: The findings demonstrate a lack of research on interventions to support patients’ decision making that takes other factors, apart from improving knowledge of trials, into account. Limited evidence exists on the effectiveness of decision support interventions to improve the experience of support in adult patients with cancer. Interventions that take patient preferences in relation to decision making and the social context of decision processes into account need to be developed and assessed.