Objective: The overall purpose of this study was to determine the effectiveness of mindfulness exercises on theory of mind in adolescent chess girls. Methods: The research design was quasi-experimental and pre-test and post-test. For this purpose, 30 girls chess players of East Azarbaijan province were selected by available means. After completing the Happe Mind Theory Questionnaire (1994), participants were randomly divided into experimental and control groups. The experimental group underwent mindfulness exercises for 8 sessions of 45 minutes and the control group did not participate in any educational activities. At the end of the training sessions, both groups completed the Happe Mind Theory (1994) questionnaire. Analysis of covariance at the level of p<0.05 was used to compare the two groups. Results: The results showed that mindfulness exercises have a positive and significant effect on theory of mind in adolescent chess girls. Conclusion: According to the findings of the present study, it can be concluded that mindfulness training can be used to improve the theory of mind and, consequently, the cognitive power of the participants.