Background: Sleep assessment in the intensive care unit (ICU) is difficult and often unreliable. The most commonly used questionnaire for assessing ICU sleep, the Richards–Campbell Sleep Scale (RCSQ), has not been tested for reliability and construct validity in the Mandarin–Taiwanese speaking population. Objective: The objective of this study was to test the construct validity and criterion validity of the traditional Chinese version of RCSQ (TC-RCSQ) in critically ill patients without physical restraint. Methods: We adopted a cross-sectional study design. Adults aged 20 years and above were recruited from a plastic surgery ICU of a medical center. The Cronbach's alpha was used to test internal consistency; the validity testing included content validity, criterion validity, and construct validity. Criterion validity was analysed by testing the association of TC-RCSQ with the Chinese version of Verran and Snyder-Halpern Sleep Questionnaire and sleep parameter of actigraphy usi