Developing a severity assessment scale for critically ill patients' thirst and conducting reliability and validity tests, aiming to provide healthcare professionals with a scientific and objective tool for assessing the level of thirst.Based on literature review and qualitative interviews, a pool of items was generated, and a preliminary scale was formed through two rounds of Delphi expert consultation. Convenience sampling was employed to select 178 ICU patients in a top-three hospital from May 2023 to October 2023 as the study subjects to examine the reliability and validity of the severity assessment scale for critically ill patients' thirst.The developed severity assessment scale for critically ill patients' thirst consists of 8 evaluation items and 26 evaluation indicators. The agreement coefficients for two rounds of expert consultation were 100% and 92.6% for the positive coefficient, and the authority coefficients were .900 and .906. Kendall's concordance coefficients were .101 and .120 (all p < .001). The overall Cronbach's α coefficient for the scale was .827. The inter-rater reliability coefficient was .910. The Item-Content Validity Index (I-CVI) ranged from .800 to 1.000, and the Scale-Content Validity Index/Average (S-CVI/Ave) was .950.The critically ill patients' thirst assessment scale is reliable and valid and can be widely used in clinical practice.The AiMi Academic Services (www.aimieditor.com) for English language editing and review services.The scale developed in this study is a simple and ICU-specific scale that can be used to assess the severity of thirst in critically ill patients. As such, the severity of thirst in critically ill patients can be evaluated quickly so that targeted interventions can be implemented according to the patient's specific disease and treatment conditions. Therefore, patient comfort can be improved, and thirst-related health problems can be prevented.© 2024 The Author(s). Journal of Clinical Nursing published by John Wiley & Sons Ltd.