Indonesia has many traditional games that can be used as a starting point in learning mathematics, i.e., social arithmetic. However, the teacher always used a conventional method such as direct teaching in teaching mathematics. Several researchers said that this method made mathematics learning boring. Therefore, this study aims to design a mathematics learning trajectory in social arithmetic using a traditional Indonesian game called the Kubuk Manuk. This game was used as the starting point in the learning process by applying the Indonesian Realistic Mathematics Education approach. The research method used is design research consisted of three stages, namely preliminary design, teaching experiment, and retrospective analysis. The results of this study indicate that the learning trajectory can be practiced using local contexts such as culture or other things easily found in the daily activities of the students. With the use of the Kubuk Manuk Indonesian traditional game, the students are stimulated to understand their knowledge of the social arithmetic concept. Additionally, the stages in the learning trajectory have essential roles in understanding the mathematics concepts of expenditure, income, profits, and loss within the trading activities.
Key words: Design research, learning trajectory, Indonesian traditional games, social arithmetic learning, mathematics concepts