Students' poor understanding of COVID-19 can contribute to an increase in the number of COVID-19 cases.. However, there is no validated instrument for measuring undergraduate student knowledge about COVID-19. This study is at the cutting edge of validating the psychometry of students' knowledge, attitudes, and practices (KAP) toward COVID-19. The assessment instrument consists of 18 items in the knowledge domain, 6 items in the attitude domain, and 12 items in the practice domain. This questionnaire underwent expert validation prior to being administered to 389 respondents. A RASCH model and Confirmatory Factor Analysis (CFA) were applied to evaluate the psychometric characteristics of the instrument. A four-factor model was tested for measurement model validity for knowledge domain, and two-factor model for attitude and practice domains by CFA. The results showed model yielded adequate goodness-of-fit values. In addition, results of RASCH model showed that the item content validity index was high. The item reliability for all the three domains was good, with a high separation index value. Thirty-six items were fitted to the model, based on recommended mean-square fit values, standardized Z-scores, and point-measure correlation coefficients. The response set in the questionnaire fit the Andrich threshold estimates well, and functioned as an appropriate model for the response category. The questionnaire thus shows excellent psychometric characteristics. Thus, this instrument can be used to measure undergraduate student KAP and can be implemented in future studies that want to assess the effectiveness of interventions to improve students' understanding of COVID-19.