This study investigated the effects of a psychophysiological intervention programmed, comprising heart rate variability biofeedback (HRV BFB) and cognitive restructuring (CR), on the self-regulation skills of an 18-year-old female squash athlete who represents the Korean national team. The participant underwent ten programme sessions in a laboratory. HRV was measured during a 10-minute baseline period of natural breathing and during BFB-guided breathing exercises. Two questionnaires, the Competitive State Anxiety Inventory-2 (CSAI-2) and the Cognitive Emotion Regulation Questionnaire (CERQ), were used to assess the participant's psychological state. Descriptive statistics were employed to analyze changes in HRV and psychological state from the initial test to the post-test. Qualitative findings indicated improvements in the participant's self-regulation skills, particularly in her ability to transform negative thoughts. In conclusion, the intervention programme shows potential in enhancing self-regulation skills to the athlete. The combination of HRV biofeedback and cognitive restructuring appears to lead to positive changes in self-regulation, which could benefit athletes in managing their psychological state and improving competitive performance. Further research and practical applications are continued needed to fully explore the programme's efficacy.