Motivation and academic self-efficacy influence learners' second language acquisition and individual differences. This study aimed to address a research gap among these variables with the Thai high school students studying Chinese as a foreign language by exploring the level of students' motivation and academic self-efficacy and investigating the relationship between motivation, academic self-efficacy, and individual differences among Thai high school students studying Chinese as a foreign language. A school in Trang, Thailand, was purposively selected as a case study due to the role of the Chinese language as a compulsory language course for the students, and 123 high school students from this school were recruited as research participants. The data collection was drawn from a quantitative method. The main research design in the quantitative part of the study was correlational. The results showed that Thai high school students had a low level of motivation in learning Chinese from the L2MSS perspectives ([x-bar]= 2.420) and had a moderate level of academic self-efficacy in Chinese language learning ([x-bar]= 2.639). Learning motivation and self-efficacy were statistically significantly correlated with students' age and Chinese language proficiency level but not statistically significantly correlated with gender, ethnicity, and duration of Chinese learning. The regression equation of motivation and self-efficacy was derived from linear regression analysis as language motivation = -0.572 + 1.134 * Self-efficacy. The results of this study also provide valuable insights for Thai Chinese teachers and native Chinese teachers to inspire students' motivation and improve academic self-efficacy in Chinese language learning.