Life in the twenty-first century requires education to integrate knowledge, attitudes, skills, and mastery of information and communication technology (ICT). An example of the skills needed in the twenty-first century is computational thinking. This study aims to develop computational thinking-skill tasks on numbers and arithmetic based on the Riau Malay culture. A research and development (R&D) method was employed with a 4-D model, which consists of four stages, namely definition, design, development, and dissemination. Furthermore, the tasks developed were validated by two mathematics education experts and tested to 33 fifth-grade students from a public school in Pekanbaru, Indonesia. The Rasch model was used to analyse the data, and the result showed that 23 valid tasks were obtained from 24 items. This implies that the tasks of assessing the computational thinking skills in the context of Riau Malay culture satisfy the valid and reliable criteria and are useful for further study.