Background: Few studies have investigated treatment environment risk factors for delirium in geriatric patients. In March 2017 a geriatric department was moved from old hospital buildings with multiple-bed rooms (old wards) to a new hospital with single-bed rooms (new wards), with no changes regarding uptake area, staff and admission criteria. Aims: The aim of this study was to investigate the risk of delirium among patients in single-bed rooms compared with multiple-bed rooms. Methods: An observational prospective study included patients aged ≥75 years admitted between 15 September 2016 and 19 March 2017 to the old wards and between 20 March and 19 December 2017 to the new wards. Exclusion criteria were terminal illness, somnolence at admission and inability to communicate in Danish. Delirium was assessed by trained nurses, nurse assistants, occupational therapists and physiotherapists every morning and evening using the Confusion Assessment Method (CAM). Results: We included 1,014 patients. Patients' characteristics were similar between patients admitted to the old wards and to the new wards. Delirium was present at admission in 105 patients, with no significant difference between the old and new wards. Patients in the new wards had a significantly reduced incidence of delirium during hospital stay compared with patients in the old wards; hazard ratio 0.66 (95%CI: 0.48-0.93, p<0.02). No difference between the old and the new wards was observed in the duration of the first delirium episode. Conclusion: We found evidence that the risk of delirium is reduced in single-bed rooms compared with multiple-bed rooms in geriatric wards.