BACKGROUND: Fidgeting devices are popular for managing ADHD, but their effectiveness remains unclear. This study investigated whether fidgeting modulates self-regulation in adults with ADHD during stress, exploring intrinsic versus extrinsic fidgeting with a fidget ball. We hypothesized that individuals with a fidget ball would demonstrate improved self-regulation under stress, reflected by higher heart rate variability (HRV) and lower anxiety levels. METHODS: Adults with ADHD were randomly assigned to a fidget ball (n = 49) or no fidget ball (n = 24) group. Physiological stress was measured via HRV and intrinsic fidgeting, that is, ankle and wrist movements, were measured via actigraphy. Physiological data were collected at baseline and during the Trier Social Stress Test (TSST). Self-rated anxiety was measured using the State Anxiety Inventory. RESULTS: Heart rate and fidgeting increased across TSST stages for both groups, with no significant differences in heart rate, HRV, or intrinsic fidgeting between groups. Notably, the fidget ball group had weaker correlations between fidgeting and HRV compared to the no fidget ball group. At baseline, the no fidget ball group showed a positive correlation between intrinsic fidgeting and HRV, suggesting an association between intrinsic fidgeting and self-regulation, however during stress the no fidget ball group showed a significant negative correlation between intrinsic fidgeting and HRV. The relationship between physiological measures and anxiety differed between groups and TSST stages. CONCLUSIONS: These results provide preliminary evidence that the fidget ball may disrupt the expected association between intrinsic fidgeting and HRV, emphasizing the need for further studies to explore its impact on self-regulation.