Objective: To determine oncology nurses’ practices about the diagnosis, prevention, and management of delirium. Background: Delirium can occur at any stage of the disease process of patients with cancer, or it can develop when the first diagnosis is learned. Study Design and Methods: A cross-sectional design was used. The study was conducted between May 10 and July 16, 2021 in Turkey with 181 nurses who worked in the oncology units of public and private hospitals and were reached by using the snowball sampling method. Data were statically analysed. Results: Of the nurses, 48.6% have a bachelor's degree and the average working years in oncology is 6.61+ 5.47, 38.1% had been making routine delirium assessments, and only 5.5% of these nurses had prior experience using a screening tool. Also, 38.1% of the nurses had difficulty diagnosing delirium, with the patient group they had difficulty diagnosing most often being patients with dementia, hypoactive delirium, and those who were sedated, respectively. The rate of nurses who worked with a patient with delirium in the last month was 31.5%, and 40.3% could not determine the type of delirium. While the nurses' definition of hyperactive delirium findings was high, their definition of hypoactive delirium findings such as lethargy (48.8%), decreased psychomotor activity (47.7%) and withdrawal (56.1%) was low. In the study, it was found that being male, having an undergraduate degree in nursing, having professional work experience, having a lower patient-to-nurse ratio, using a screening tool, having experience working with patients diagnosed with delirium, and having positive views on delirium treatment had a significant impact on nurses' routine follow-up of delirium (p<0.05). In addition, it was determined that those who had experience working with patients diagnosed with delirium had less difficulty diagnosing delirium (p<0.05). Also, 82.4% of the nurses stated that they first applied pharmacological treatment after the diagnosis of delirium and that they most frequently used haloperidol (35%), benzodiazepine (29.8%) and dexmedetomidine (22.8%). Although nurses were self-confident about the management of delirium, they thought that it was difficult to prevent and treat delirium in patients with cancer. Conclusions: Although approximately one-third of the nurses had cared for patients diagnosed with delirium in the last month, the rate of using a measurement tool for routine delirium screening and diagnosis was low. Implications for research, policy, and practice: It was concluded that it was important to support the education of nurses with case examples in in-service or certificate programs, workforce planning should be made according to patient/nurse ratios, and evidence-based care procedures should be prepared and implemented so that oncology nurses could do routine follow-up of delirium in patients diagnosed with cancer, use a valid and reliable screening tool, and manage effectively and appropriately delirium. What is already known about this topic? Although the incidence of delirium is high in cancer patients, it is not always diagnosed appropriately or managed effectively. Despite the factors limiting diagnosis and treatment in cancer patients, 40% of delirium cases can be prevented. Delirium is a syndrome that can be managed with pharmacological and non-pharmacological applications. What this paper adds: In the diagnosis and management of delirium, whether nurses use screening tools and do routine follow-up, the level of their education, their experience working with patients diagnosed with delirium, the number of patients they provide care for, their self-confidence, and their positive attitudes towards delirium are factors that impact on the diagnosis and outcomes. Nurses should be supported with case-based education that will increase their knowledge and skills regarding the diagnosis and management of delirium in patients diagnosed with cancer. This approach will increase nurses' experience before they encounter a patient with delirium. For effective diagnosis and management of delirium, workforce planning should consider patient/nurse ratios in oncology units as a workload management and patient safety measure. It is recommended to prepare and implement institutional care procedures for the diagnosis, management and follow-up of delirium in oncology units.