Study aim: Determining the effectiveness of mindfulness training on competitive anxiety and physiological indices in martial arts athletes. Design: A randomized, unblinded, parallel-group clinical trial with a pretest-posttest design will be conducted on 30 Ninjutsu athletes allocated (1:1) to intervention and control groups via permuted block randomization. Settings and conduct: This parallel-group, randomized clinical trial with a pretest-posttest design will be conducted in 2026 at Cheshm Oghab Ninjutsu Academy to investigate the effectiveness of mindfulness training on competitive anxiety and physiological indicators in martial arts athletes. Thirty eligible athletes will be selected via purposive sampling and randomly allocated (1:1 ratio) to an intervention group (mindfulness training) and a control group using permuted block randomization. Due to the nature of the psychological intervention, this study will be conducted without blinding of the participants and researchers. Participants/Inclusion and exclusion criteria: Inclusion criteria: Martial arts athletes in Ninjutsu; Having at least one year of activity experience in the martial art of Ninjutsu; Informed consent to participate in the research. Exclusion criteria: History of participation in mindfulness workshops or courses in the past; History of hospitalization in a hospital due to severe psychiatric disorders. Intervention groups: Intervention group: In addition to regular training, will receive 8 weeks (1 session/week) of Mindfulness-Based Stress Reduction (MBSR) training. Control group: Will continue regular training only without any psychological intervention (will receive training after the study). Main outcome variables: Competitive anxiety (CSAI-2R questionnaire); serum cortisol level; heart rate and blood pressure (digital monitor), which will be assessed at pre-test and post-test.