Background: Constipation is a functional gastrointestinal disorder that presents with signs and symptoms, which are typically assessed subjectively. Various measurement scales, such as the Constipation Assessment Scale (CAS), are commonly used to evaluate constipation among the general population. However, the instruments should be culturally and contextually relevant in adult psychiatric patients to generate valid and reliable evidence. Purpose: This study aimed to cross-culturally adapt and psychometrically validate the traditional Chinese version of the CAS among adult psychiatric patients in Hong Kong. Method: Using the Brislin protocol and Consensus-based Standards for the selection of health Measurement Instruments (COSMIN) guidelines, the CAS was translated into traditional Chinese and tested for internal consistency, test–retest reliability, content validity, and construct validity among psychiatric patients in Hong Kong. Results: The CAS was successfully translated into CAS-TC. The CAS-TC version demonstrated good content validity (scale level CVI = 97%), internal consistency (Cronbach’s alpha = 0.79), and test–retest reliability (ICC = 0.722 [95% CI, 0.587–0.812]). The CAS-TC showed a two-factor loading for the construct validity, which explained 54% of the total variance. Conclusions: The CAS-TC is valid and reliable and can be employed to assess constipation among adult psychiatric patients.