BackgroundDelirium is a common yet challenging condition in older hospitalized patients, associated with various adverse outcomes. Environmental factors, such as room changes, may contribute to the development or severity of delirium. Most previous research has focused on preventing and reducing this condition by addressing risk factors and facilitating reorientation during hospital stay. ObjectiveWe aimed to systematically develop a complex intervention to prevent delirium in older hospitalized patients by optimizing discharge and transfer processes and involving caregivers during and after these procedures. The intervention combines stakeholder and expert opinions, evidence, and theory. This article provides guidance and inspiration to research groups in developing complex interventions according to the recommendations in the Medical Research Council framework for complex interventions. Design and methodsA stepwise multi-method study was conducted. The preparation phase included analysis of the context and current practice via focus groups. Based on these results, an expert workshop was organized, followed by a Delphi survey. Finally, the intervention was modeled and a program theory was developed, including a logic model. ResultsA complex intervention was developed in an iterative process, involving healthcare professionals, delirium experts, researchers, as well as caregiver and patient representatives. The key intervention component is an 8-point-program, which provides caregivers with recommendations for preventing delirium during the transition phase and in the post-discharge period. Information materials (flyers, handbook, videos, posters, defined “Dos and Don'ts”, discharge checklist), training for healthcare professionals, and status analyses are used as implementation strategies. In addition, roles were established for gatekeepers to act as leaders, and champions to serve as knowledge multipliers and trainers for the multi-professional team in the hospitals. ConclusionsThis study serves as an example of how to develop a complex intervention. In an additional step, the intervention and implementation strategies will be investigated for feasibility and acceptability in a pilot study with an accompanying process evaluation. Tweetable abstractDelirium prevention can benefit from optimizing discharge and transfer processes and involving caregivers of older patients in these procedures. Study registrationDRKS00017828, German Register of Clinical Studies, date of registration 17.09.2019.