The pervasive use of smartphones has significantly contributed to distracted driving, a leading cause of road traffic accidents globally. This study investigates the behavioural intentions and patterns of smartphone use while riding among motorcyclists in Vietnam, integrating the Theory of Planned Behaviour (TPB) with the Stimuli-Organism-Response (SOR) framework to encompass factors such as riding exposure and time pressure. A questionnaire survey was conducted, gathering data from 1,051 young motorcyclists. Using Partial Least Squares Structural Equation Modelling (PLS-SEM), the study identifies high levels of smartphone engagement during riding, driven primarily by Perceived Behavioural Control (PBC), which exhibited a stronger influence on behaviour than Attitudes and Social Norms. Notably, time pressure significantly enhanced the intention to use smartphones, suggesting that riding under time constraints could exacerbate the risk of distracted riding incidents. The findings highlight critical implications for road safety interventions and policy formulation, emphasising the need for targeted educational programmes and stricter enforcement measures to mitigate smartphone-induced distractions among motorcyclists at a higher risk of traffic accidents. The study contributes to understanding distracted riding behaviours in motorcycle-dominant regions, providing a foundation for future research and preventive strategies.