Older adults are considered a vulnerable group due to physical, emotional, and cognitive changes, and anxiety disorders can exacerbate their mental health challenges. Illness anxiety disorder, characterized by a persistent fear of having a serious illness despite medical reassurance, has considerable psychological consequences for this population. The present study aimed to investigate the effectiveness of mindfulness-based stress reduction (MBSR) on the psychological health of an older adult diagnosed with illness anxiety disorder. A single-subject experimental design with an A-B-A baseline scheme was employed. Among older adults referred to a psychiatric clinic in October 2023, one participant was selected based on inclusion criteria using purposive sampling. The MBSR intervention was delivered over eight 90-minute sessions. The Goldberg General Health Questionnaire (GHQ-28) was used as the research instrument. Data were analyzed using visual inspection at both within-phase and between-phase levels using SPSS and Excel software. The results indicated that the mindfulness intervention enhanced the participant's psychological health, with scores during the eight intervention sessions and the follow-up phase showing a significant decrease (reflecting improved health) compared to the baseline. The Percentage of Non-overlapping Data (PND) was 75%, indicating the clinical effectiveness of the intervention. It appears that mindfulness, by fostering non-judgmental awareness and reducing rumination, may serve as an effective strategy for improving mental health in older adults with illness anxiety disorder.