Introduction Hypertrophic cardiomyopathy (HCM) patients often present an enhanced arrhythmogenicity that can lead to lethal arrhythmias, especially during exercise. Recent studies have indicated an abnormal response of HCM cardiomyocytes to β-adrenergic receptor stimulation (β-ARS), with prolongation of their action potential rather than shortening. Aims This work aims to investigate the ionic mechanisms underlying the HCM abnormal response to β-ARS and its possible proarrhythmic role using human-based experimental and computational methodologies. The changes induced by the HCM remodelling and β-ARS in the electrocardiogram (ECG) of the patient will also be evaluated. Material and Methods The latest models of human ventricular electrophysiology and β-ARS were integrated and calibrated using experimental measurements of human adult cardiomyocytes from control and HCM patients. A population of models approach was used to determine the principal ionic mechanisms underlying the main changes of cellular response in HCM under β-ARS. Organ level simulations, with and without β-ARS response, were also performed to study the evolution of the dispersion of repolarization. Simulated 12-lead ECGs were computed at clinically standard lead locations. Results The developed in silico models of β-ARS capture the behaviour observed in the experimental data, including the aberrant response of HCM cardiomyocytes to β-ARS. A reduced increase of potassium currents under β-ARS was identified as the main mechanism of action potential prolongation in HCM and therefore, the main factor under the QT interval prolongation in the ECG. Dispersion of repolarisation between healthy and HCM tissue was increased upon β-ARS, while transmural dispersion in HCM tissue was reduced. Conclusions Our results identify causal relationships between the HCM phenotype and its arrhythmogenic response to β-ARS through the downregulation of potassium currents. The developed model offers new insights into the mode of function of β-blockers in HCM and on the ionic mechanisms underlying proarrhythmic events in the disease.