Elderly people with cognitive impairment are at risk for under-treatment of pain due to their inability to communicate. Poor knowledge and attitudes of nurses toward pain in this population may result in inadequate pain assessment. This study used a descriptive correlational design to develop and validate a tool to assess nurses' knowledge, attitudes, and reported practice of pain assessment in cognitively impaired elderly patients in acute care settings. The Tool for Evaluating the ways Nurses Assess Pain (TENAP) has two sections: (1) nurses' knowledge and attitudes about pain assessment and management and (2) two vignettes to assess reported practice. Content validity was established by an expert panel of three geriatric-trained nurse clinicians, and pilot tested with a convenience sample of 10 nurses. The psychometric properties were tested with a sample of 263 Registered and Enrolled nurses working in medical wards of two public hospitals in Singapore. The final version of TENAP comprised 29 items. Content validity index ranged from 0.84 to 1.00. The scale took 10 to 15 minutes to complete and items were easily understood. Results from the factor analysis suggested that Section A demonstrated one factor (13 items) while Section B had two distinct factors (16 items), one for each vignette, supporting construct validity of the scale. Cronbach's alphas for all factors were acceptable. TENAP was feasible, valid, and reliable for assessing nurses' knowledge, attitudes, and reported practice of pain assessment in cognitively-impaired elderly patients. Further testing of the tool with a larger sample of nurses in other practice contexts is needed.