Aims: To examine hospital nurses’ perception of their actual and potential contribution to shared decision-making about life-prolonging treatment and their perception of the pre-conditions for such a contribution. Design: A qualitative interview study. Methods: Semi-structured face-to-face interviews were conducted with 18 hospital nurses who were involved in care for patients with life-threatening illnesses. Data were collected from October 2018-January 2019. The interviews were recorded, transcribed verbatim and analysed using thematic analysis by two researchers. Results: Nurses experienced varying degrees of influence on decision-making about life-prolonging treatment. Besides, we identified different points of contact in the treatment trajectory at which nurses could be involved in treatment decision-making. Nurses’ descriptions of behaviours that potentially contribute to shared decision-making were classified into three roles as follows: checking the quality of a decision, complementing shared decision-making and facilitating shared decision-making. Pre-conditions for fulfilling the roles identified in this study were: (a) the transfer of information among nurses and between nurses and other healthcare professionals; (b) a culture where there is a positive attitude to nurses' involvement in decision-making; (c) a good relationship with physicians; (d) knowledge and skills; (e) sufficient time; and (f) a good relationship with patients. Conclusion: Nurses described behaviour that reflected a supporting role in shared decision-making about patients’ life-prolonging treatment, although not all nurses experienced this involvement as such. Nurses can enhance the shared decision-making process by checking the decision quality and by complementing and facilitating shared decision-making. Impact: Nurses are increasingly considered instrumental in the shared decision-making process. To facilitate their contribution, future research should focus on the possible impact of nurses’ involvement in treatment decision-making and on evidence-based training to raise awareness and offer guidance for nurses on how to adopt this role.