Background Today, computational thinking (CT), which is considered to be a form of literacy, has taken its place in the ICT curriculum of many countries at the K‐12 level. Therefore, there is a need for more evidence with regards to a theoretical and practical understanding of CT skills’ development of K‐12 students. Objectives The purpose of the study was to investigate the effect of an unplugged coding course on primary school students’ development of CT skills, differences in their CT skills in terms of socio‐demographics such as gender, computer ownership, daily computer use, and home Internet access, and the relationship between their CT and 21st century skills. Methods The research was based a quasi‐experimental design with one‐group pretest‐posttest with follow‐up. The CT skills of 212 third and fourth grade students at a public primary school of Turkey were measured with a CT Skills Test before, after, and about ten weeks following having attended an unplugged coding course. Results and Conclusions The results of the study showed that the unplugged coding course statistically significantly improved the participants’ CT skills, specifically in algorithmic design, abstraction, evaluation, decomposition, and generalization. Moreover, the findings indicated that primary school students’ CT skills were not associated with their socio‐demographics. In addition, the students’ CT skills were found to be positively and statistically significantly correlated with their collaboration and communication skills. Implications This study contributes to understanding of the effects of unplugged activities on the development of primary school students’ CT skills, which is beneficial to teaching practices for CT skills in the primary education.