Background <p>Delirium affects up to two-thirds of palliative care unit (PCU) inpatients, causing distress for patients, families, and staff, and contributing to complex care needs and increased healthcare resource use. Although evidence-based guidelines exist, their consistent implementation in PCUs is limited, and implementation strategies to support guideline-adherent care have not been rigorously evaluated in this setting. Creating Learning Environments for Compassionate Care-Palliative Delirium (CLECC-Pal Delirium) is a co-designed multi-component implementation strategy to help embed guideline-adherent delirium care within routine PCU practice. This cluster randomised controlled trial (cRCT) evaluates the effectiveness and cost-effectiveness of the implementation strategy using an implementation-to-target design, with embedded economic and process evaluations.</p> Methods <p>Adaptive implementation-to-target, Type III hybrid effectiveness-implementation parallel group cRCT across 20 PCUs (10 per arm, ~ 50 patient records per cluster at each timepoint) in the UK, randomised 1:1 to CLECC-Pal Delirium or usual care. Randomisation is stratified by unit size, education provision, and provider type (NHS/Charity). Sites will receive tailored support to achieve predefined implementation-readiness criteria prior to data collection. The primary outcome is the proportion of each admission’s inpatient days affected by delirium (delirium days), measured retrospectively using a validated chart-based method adapted for palliative care. Secondary outcomes include adherence to delirium care guidelines, patient symptom burden, functional status, and incremental cost-effectiveness. The sample size (1200 admission episodes with delirium) provides 92.3% power to detect a 12% absolute reduction in delirium days, based on an assumed intracluster correlation coefficient (ICC) of 0.03 (α = 0.05). Mixed-effects regression models, with clustering at the PCU level, will be employed to analyse the primary outcome, adjusting for stratification factors and other prespecified covariates, and conducted on an intention-to-treat basis. Blinded statisticians and health economists will conduct analyses. The process evaluation, guided by Normalisation Process Theory, will examine implementation fidelity and how, why, and in what contexts the strategy is integrated into practice.</p> Discussion <p>This trial will provide evidence on the clinical- and cost-effectiveness of an implementation strategy to improve delirium care in PCUs, strengthening the implementation evidence base in a setting where implementation research is limited.</p> Trial registration <p>ISRCTN23678136; registered 14th April 2025.</p>