This study aimed to develop a supplementary mathematics textbook on traditional game-based computational thinking for elementary school students. This textbook was developed using a research and development model consisting of five stages: analysis, design, development, implementation, and evaluation (ADDIE). The mathematical contexts of the textbook were based on four traditional Indonesian games integrated into four computational thinking skills: decomposition, pattern recognition, abstraction, and algorithms. The supplementary mathematics textbook was developed and validated by experts in mathematics and culture. The textbook was tested with 20 fourth-grade students. The findings of this study indicated that the supplementary mathematics textbook of traditional game-based computational thinking has high usability as rated by students. The students gave positive feedback for use of the textbook in the classroom because of its connection to culture and games. Therefore, the study enriches learning materials on culturally integrated computational thinking skills for elementary school students.