The purposes of this study were to determine essential competency components for psychologists providing assessment services in Taiwan and to develop the Psychological Assessment Competency Scale (PACS), a new instrument for measuring this concept. In Study 1, we derived 13 coding categories from both the 8-factor psychological assessment competency (PAC) model of the Psychological Assessment Work Group (PAWG) and the interview data of 46 licensed psychologists in Taiwan. These categories represent essential PAC components for psychologists providing assessment services in Taiwan and were used as the theoretical framework of the PACS for serving this population. In Study 2, we recruited 235 licensed psychologists in Taiwan to investigate the psychometric properties of the PACS. The results of exploratory factor and confirmatory factor analyses revealed that the PACS contained 9 factors that explained 65.82% of the total variance. Cronbach’s alpha values of the PACS total and subscale scores were all above .7. The results of our study partially support the PAWG 8-factor model and are consistent with a major assumption of the Cube model. In other words, competencies should be measured according to both foundational and functional perspectives. The findings of this study also support the importance of 4 foundational competencies that enable psychologists to provide assessment services in Taiwan. Implications of the study and suggestions were also discussed.